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LIE. 


XL  THE  SPECIAL  SENSES. 


A.   VISION. 

The"  Physiology  of  Vision. — The  eye  is  the  organ  by  means  of  which 
certain  vibrations  of  the  luminiferous  ether  are  enabled  to  affect  our  conscious- 
ness, producing  the  sensation  which  we  call  "  light."  Hence  the  essential  part 
of  an  organ  of  vision  is  a  substance  or  an  apparatus  which,  on  the  one  hand, 
is  of  a  nature  to  be  stimulated  by  waves  of  light,  and,  on  the  other,  is  so  con- 
nected with  a  nerve  that  its  activity  causes  nerve-impulses  to  be  transmitted  to 
the  nerve-centres.  Any  animal  in  which  a  portion  of  the  ectoderm  is  thus 
differentiated  and  connected  may  be  said  to  possess  an  eye — i.  e.  an  organ 
through  which  the  animal  may  consciously  or  unconsciously  react  to  the  exist- 
ence of  light  around  it.1  But  the  human  eye,  as  well  as  that  of  all  the  higher 
animals,  not  only  informs  us  of  the  existence  of  light,  but  enables  us  to  form 
correct  ideas  of  the  direction  from  which  the  light  conies  and  of  the  form,  color, 
and  distance  of  the  luminous  body.  To  accomplish  this  result  the  substance 
sensitive  to  light  must  form  a  part  of  a  complicated  piece  of  apparatus  capable 
of  very  varied  adjustments.  The  eye  is,  in  other  words,  an  optical  instrument, 
and  its  description,  like  that  of  all  optical  instruments,  includes  a  consideration 
of  its  mechanical  adjustments  and  of  its  refracting  media. 

Mechanical  Movements. — The  first  point  to  be  observed  in  studying  the 
movements  of  the  eye  is  that  they  are  essentially  those  of  a  ball-and-socket 
joint,  the  globe  of  the  eye  revolving  freely  in  the  socket  formed  by  the  capsule 
of  Tenon  through  a  horizontal  angle  of  almost  88°  and  a  vertical  angle  of  about 
80°.  The  centre  of  rotation  of  the  eye  (which  is  not,  however,  an  absolutely 
fixed  point)  does  not  coincide  with  the  centre  of  the  eyeball,  but  lies  a  little 
behind  it.  It  is  rather  farther  forward  in  hypermetropic  than  in  myopic  eyes. 
The  movements  of  the  eye,  especially  those  in  a  horizontal  direction,  are  sup- 
plemented by  the  movements  of  the  head  upon  the  shoulders.  The  combined 
eye  and  head  movements  are  in  most  persons  sufficiently  extensive  to  enable 
the  individual,  without  any  movement  of  the  body,  to  receive  upon  the  lateral 
portion  of  the  retina  the  image  of  an  object  directly  behind  his  back.  The 
rotation  of  the  eye  in  the  socket  is  of  course  easiest  and  most  extensive  when 
the  eyeball  has  an  approximately  spherical  shape,  as  in  the  normal  or  emme- 
tropic  eye.  When  the  antero-posterior  diameter  is  very  much  longer  than  those 

1  In  certain  of  the  lower  orders  of  animals  no  local  differentiations  seem  to  have  occurred, 
and  the  whole  surface  of  the  body  appears  to  be  obscurely  sensitive  to  light.     See  Nagel :  Der 
Lichtxinn  augerdoser  Thiere,  Jena,  1896. 
744 


THE  SENSE    OF    VISION.  745 

at  right  angles  to  it,  as  in  extremely  myopic  or  short-sighted  eyes,  the  rotation 
of  the  eyeball  may  be  considerably  limited  in  its  extent.  In  addition  to  the 
movements  of  rotation  round  a  centre  situated  in  the  axis  of  vision,  the  eye- 
ball may  be  moved  forward  and  backward  in  the  socket  to  the-«xtent  of  about 
one  millimeter.  This  movement  may  be  observed  whenever  the  eyelids  are 
widely  opened,  and  is  supposed  to  be  effected  by  the  simultaneous  contraction  of 
both  the  oblique  muscles.  A  slight  lateral  movement  has  also  been  described. 
The  movements  of  the  eye  will  be  best  understood  when  considered  as 
referred  to  three  axes  at  right  angles  to  each  other  and  passing  through  the 
centre  of  rotation  of  the  eye.  The  first  of  these  axes,  which  may  be  called 
the  longitudinal  axis,  is  best  described  as  coinciding  with  the  axis  of  vision 
when,  with  head  erect,  we  look  straight  forward  to  the  distant  horizon ;  the 
second,  or  transverse,  axis  is  defined  as  a  line  passing  through  the  centres  of 
rotation  of  the  two  eyes ;  and  the  third,  or  vertical,  axis  is  a  vertical  line  nec- 
essarily perpendicular  to  the  other  two  and  also  passing  through  the  centre  of 
rotation.  When  the  axis  of  vision  coincides  with  the  longitudinal  axis,  the  eye 
is  said  to  be  in  the  primary  position.  When  it  moves  from  the  primary  posi- 
tion by  revolving  around  either  the  transverse  or  the  vertical  axis,  it  is  said  to 
assume  seeondary  positions.  All  other  positions  are  called  tertiary  positions, 
and  are  reached  from  the  primary  position  by  rotation  round  an  axis  which 
lies  in  the  same  plane  as  the  vertical  and  horizontal  axis — i.  e.  in  the  "  equato- 
rial plane "  of  the  eye.  When  the  eye  passes  from  a  secondary  to  a  tertiary 
position,  or  from  one  tertiary  position  to  another,  the  position  assumed  by  the 
eye  is  identical  with  that  which  it  would  have  had  if  it  had  reached  it  from 
the  primary  position  by  rotation  round  an  axis  in  the  equatorial  plane.  In 
other  words,  every  direction  of  the  axis  of  vision  is  associated  with  a  fixed 
position  of  the  whole  eye — a  condition  of  the  greatest  importance  for  the  easy 
and  correct  use  of  the  eyes.  A  rotation  of  the  eye  round  its  antero-posterior 
axis  takes  place  in  connection  with  certain  movements,  but  authorities  diifer 
with  regard  to  the  direction  and  amount  of  this  rotation. 

Muscles  of  the  Eye. — The  muscles  of  the  eye  are  six  in  number — viz : 
the  superior,  inferior,  internal  and  external  recti,  and  the  superior  and  inferior 
oblique.  This  apparent  superfluity  of  muscles  (for  four  muscles  would  suffice 
to  turn  the  eye  in  any  desired  direction)  is  probably  of  advantage  in  reducing 
the  amount  of  muscular  exertion  required  to  put  the  eye  into  any  given  posi- 
tion, and  thus  facilitating  the  recognition  of  slight  differences  of  direction,  for, 
according  to  Fechner's  psycho-physic  law  the  smallest  perceptible  difference  in 
a  sensation  is  proportionate  to  the  total  amount  of  the  sensation.  Hence  if  the 
eye  can  be  brought  into  a  given  position  by  a  slight  muscular  effort,  a  change 
from  that  position  will  be  more  easily  perceived  than  if  a  powerful  effort  were 
necessary. 

Each  of  the  eye-muscles,  acting  singly,  tends  to  rotate  the  eye  round  an  axis 
which  may  be  called  the  axis  of  rotation  of  that  muscle.  Now,  none  of  the 
muscles  have  axes  of  rotation  lying  exactly  in  the  equator  of  the  eye — i.e. 
in  a  plane  passing  through  the  centre  of  rotation  perpendicular  to  the  axis 


746  ^4^  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

of  vision.1  But  all  movements  of  the  eye  from  the  primary  position  take  place, 
as  we  have  seen,  round  an  axis  lying  in  this  plane.  Hence  all  such  movements 
must  be  produced  by  more  than  one  muscle,  and  this  circumstance  also  is  prob- 
ably of  advantage  in  estimating  the  extent  and  direction  of  the  movement.  In 
this  connection  it  is  interesting  to  note  that  the  eye-muscles  have  an  exception- 
ally abundant  nerve-supply — a  fact  which  it  is  natural  to  associate  with  their 
power  of  extremely  delicate  adjustment.  It  has  been  found  by  actual  count 
that  in  the  muscles  of  the  human  eye  each  nerve-fibre  supplies  only  two  or  three 
muscle-fibres,  while  in  the  muscles  of  the  limbs  the  ratio  is  as  high  as  1  to 
40-1 25.2 

Although  each  eye  has  its  own  supply  of  muscles  and  nerves,  yet  the  two 
eyes  are  not  independent  of  each  other  in  their  movements.  The  nature  of 
their  connections  with  the  nerve-centres  is  such  that  only  those  movements  are, 
as  a  rule,  possible  in  which  both  axes  of  vision  remain  in  the  same  plane.  This 
oondition  being  fulfilled,  the  eyes  may  be  together  directed  to  any  desired  point 
above,  below,  or  at  either  side  of  the  observer.  The  axes  may  also  be  con- 
verged, as  is  indeed  necessary  in  looking  at  near  objects,  and  to  facilitate  this 
convergence  the  internal  recti  muscles  are  inserted  nearer  to  the  cornea  than  the 
other  muscles  of  the  eye.  Though  in  the  ordinary  use  of  the  eyes  there  is  never 
any  occasion  ^to  diverge  the  axes  of  vision,  yet  most  persons  are  able  to  effect  a 
divergence  of  about  four  degrees,  as  shown  by  their  power  to  overcome  the  ten- 
dency to  double  vision  produced  by  holding  a  prism  in  front  of  one  of  the  eyes. 
The  nervous  mechanism  through  which  this  remarkable  co-ordination  of  the 
muscles  of  the  two  eyes  is  effected,  and  their  motions  limited  to  those  which 
are  useful  in  binocular  vision,  is  not  completely  understood,  but  it  is  supposed 
to  have  its  seat  in  part  in  the  tubercula  quadrigemina,  in  connection  with  the 
nuclei  of  origin  of  the  third,  fourth,  and  sixth  cranial  nerves.  Its  disturbance 
by  disease,  alcoholic  intoxication,  etc.  causes  strabismus,  confusion,  dizziness, 
and  double  vision. 

A  nerve  termination  sensitive  to  light,  and  so  arranged  that  it  can  be  turned 
in  different  directions,  is  sufficient  to  give  information  of  the  direction  from 
which  the  light  comes,  for  the  contraction  of  the  various  eye-muscles  indicates, 
through  the  nerves  of  muscular  sense,  the  position  into  which  the  eye  is  nor- 
mally brought  in  order  to  best  receive  the  luminous  rays,  or,  in  other  words, 
the  direction  of  the  luminous  body.  The  eye,  however,  informs  us  not  only  of 
the  direction,  but  of  the  form  of  the  object  from  which  the  light  proceeds;  and 
to  understand  how  this  is  effected  it  will  be  necessary  to  consider  the  refracting 
media  of  the  eye  by  means  of  which  an  optical  image  of  the  luminous  object 
is  thrown  upon  the  expanded  termination  of  the  optic  nerve — viz.  the  retina. 

Dioptric  Apparatus  of  the  Eye. — For  the  better  comprehension  of  this 
portion  of  the  subject  a  few  definitions  in  elementary  optics  mny  be  given.  A 

1  The  axes  of  rotation  of  the  internal  and  external  recti,  however,  deviate  l>ut  slightly  from 
the  equatorial  plane. 

2  P.  Tergast :  "  Ueber  das  Verhiiltniss  von  Nerven  und  Muskelu,"  Arclu'r  fur  mikr.  Anal.. 
ix.  36-46. 


THE   SENSE    OF    VISION. 


'41 


dioptric  system  in  its  simplest  form  consists  of  two  adjacent  media  which  have 
different  indices  of  refraction  and  whose  surface  of  separation  is  the  segment 
of  a  sphere.  A  line  joining  the  middle  of  the  segment  with  the  centre  of  the 
sphere  and  prolonged  in  either  direction  is  called  the  axis  of  the  system.  Let 
the  line  APE  in  Figure  213  represent  in  section  such  a  spherical  surface  the 


B 

FIG.  213.— Diagram  of  simple  optical  system  (after  Foster). 

centre  of  which  is  at  N,  the  rarer  medium  being  to  the  left  and  the  denser  me- 
dium to  the  right  of  the  line.  Any  ray  of  light  which,  in  passing  from  the 
rarer  to  the  denser  medium,  is  normal  to  the  spherical  surface  will  be  unchanged 
in  its  direction — i.  e.  will  undergo  no  refraction.  Such  rays  are  represented  by 
the  lines  0  Py  MD,  and  Mf  E.  If  a  pencil  of  rays  having  its  origin  in  the  rarer 
medium  at  any  point  in  the  axis  falls  upon  the  spherical  surface,  there  will  be 
one  ray — viz.  the  one  which  coincides  with  the  axis  of  the  system,  which  will 
pass  into  the  second  medium  unchanged  in  its  direction.  This  ray  is  called 
the  principal  ray  (OP),  and  its  point  of  intersection  (P)  with  the  spherical 
surface  is  called  the  principal  point.  The  centre  of  the  sphere  (N)  through 
which  the  principal  ray  necessarily  passes  is  called  the  nodal  point.  All  the 
other  rays  in  the  pencil  are  refracted  toward  the  principal  ray  by  an  amount 


FIG.  214.— Diagram  to  show  method  of  finding  principal  foci  (Neumann). 

which  depends,  for  a  given  radius  of  curvature,  upon  the  difference  in  the 
refractive  power  of  the  media,  or,  in  other  words,  upon  the  retardation  of  light 
in  passing  from  one  medium  to  the  other.  If  the  incident  rays  have  their 
origin  at  a  point  infinitely  distant  on  the  axis — /.  e.  if  they  are  parallel  to  each 
other — they  will  all  be  refracted  to  a  point  behind  the  spherical  surface  known 


748  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

as  the  principal  focus ,  F.  There  is  another  principal  focus  (Ff)  in  front  of  the 
spherical  surface — viz.  the  point  from  which  diverging  incident  rays  will  be 
refracted  into  parallelism  on  passing  the  spherical  surface,  or,  in  other  words, 
the  point  at  which  parallel  rays  coming  from  the  opposite  direction  will  be 
brought  to  a  focus.  The  position  of  these  two  principal  foci  may  be  deter- 
mined by  the  construction  shown  in  Figure  214.  Let  CA  Cf  represent  a  sec- 
tion of  a  spherical  refracting  surface  with  the  axis  A  N9  the  nodal  point  Nf  and 
the  principal  point  A.  The  problem  is  to  find  the  foci  of  rays  parallel  to  the 
axis.  Erect  perpendiculars  at  A  and  N.  Set  off  on  each  perpendicular  dis- 
tances No,  Np,  A  of,  Apr  proportionate  to  the  rapidity  of  light  in  the  two  media 
(e.  g.  2  :  3).  The  points  where  the  lines  p'  o  and  p  o'  prolonged  will  cut  the 
axis  are  the  two  principal  foci  F  and  F' — i.  e.  the  points  at  which  parallel  rays 
coming  from  either  direction  are  brought  to  a  focus  after  passing  the  spherical 
refracting  surface.  If  the  rays  are  not  parallel,  but  diverging — i.  e.  coming 
from  an  object  at  a  finite  distance — the  point  where  the  rays  will  be  brought  to 
a  focus,  or,  in  other  words,  the  point  where  the  optical  image  of  the  luminous 
object  will  be  formed,  may  be  determined  by  a  construction  which  combines 
any  two  of  the  three  rays  whose  course  is  given  in  the  manner  above  described. 
Thus  in  Figure  215  let  AN  be  the  axis,  and  F  and  Ff  the  principal  foci  of 


°\ 

FIG.  215.— Diagram  to  show  method  of  finding  conjugate  foci. 

the  spherical  refracting  surface  CA  Cf,  with  a  nodal  point  at  N.  Let  B  be 
the  origin  of  a  pencil  of  rays  the  focus  of  which  is  to  be  determined.  Draw 
the  line  B  C  representing  the  course  of  an  incident  ray  parallel  to  the  axis. 
This  ray  will  necessarily  be  refracted  through  the  focus  F,  its  course  being 
represented  by  the  line  CF  and  its  prolongation.  Similarly,  the  incident  ray 
passing  through  the  focus  F'  and  striking  the  spherical  surface  at  C'  will,  after 
refraction,  be  parallel  to  the  axis — i.  e.  it  will  have  the  direction  Cf  b.  The 
principal  ray  of  the  pencil  will  of  course  pass  through  the  spherical  surface  and 
the  nodal  point  N  without  change  of  direction.  These  three  rays  will  come 
together  at  the  same  point  6,  the  position  of  which  may  be  determined  by  con- 
structing the  course  of  any  two  of  the  three.  The  points  B  and  b  are  called 
conjugate  foci,  and  are  related  to  each  other  in  such  a  way  that  an  optical  image 
is  formed  at  one  point  of  a  luminous  object  situated  at  the  other.  When  the 
rays  of  light  pass  through  several  refracting  surfaces  in  succession  their  course 
may  be  determined  by  separate  calculations  for  each  surface,  a  process  which 
is  much  simplified  when  the  surfaces  are  "  centred  " — i.  e.  have  their  centres 
of  curvature  lying  in  the  same  axis,  as  is  approximately  the  case  in  the  eye. 

Refracting-  Media  of  the  Eye. — Rays  of  light  in  passing  through  the  eye 
penetrate  seven  different  media  and  are  refracted  at  seven  surfaces.    The  media 


THE  SENSE    OF    VISION.  749 

are  as  follows :  layer  of  tears,  cornea,  aqueous  humor,  anterior  capsule  of  lens 
lens,  posterior  capsule  of  lens,  vitreous  humor.     The  surfaces  are  those  which 
separate  the  successive  media  from  each  other  and  that  which  separates  the  tear 
layer  from  the  air.     For  purposes  of  practical  calculation  thejmmber  of 
faces  and  media  may  be  reduced  to  three.     In  the  first  place,  the  layer  of  tears 
which  moistens  the  surface  of  the  cornea  has  the  same  index  of  refraction  as 
the  aqueous  humor.     Hence  the  index  of  refraction  of  the  cornea  may  be  left 
out  of  account,  since,  having  practically  parallel  surfaces  and  being  bounded 
on  both  sides  by  substances  having  the  same  index  of  refraction,  it  does  not 
influence  the  direction  of  rays  of  light  passing  through  it. 
reason  objects  seen  obliquely  through  a  window  aPI>ear  in  their  true  directi 
the  refraction  of  the  rays  of  light  on  entering  the  glass  being  equal  in  amount 
and  opposite  in  direction  to  that  which  occurs  in  leaving  it.     For  purpose 
optical  calculation  we  may,  therefore,  disregard  the  refraction  of  the  cornea 
(which   moreover,  does  not  differ  materially  from  that  of  the  aqueous  humor), 
and  imagine  the  aqueous  humor  extending  forward  to  the  anterior  surface  of 
the  layer  of  tears  which  bathes  the  corneal  epithelium.    Furthermore,  the  cap- 
sule of  the  lens  has  the  same  index  of  refraction  as  the  outer  layer  of  the  lens 
itself,  and  for  optical  purposes  may  be  regarded  as  replaced  by  it. 
the  optical  apparatus  of  the  eye  may  be  regarded  as  consisting  of  the  : 
lowing  three  refracting  media:    Aqueous  humor,  index  of  refraction   1.35 
lens,  average  index  of  refraction  1.45;  vitreous  humor,  index  of  refraction 
1  33      The  surfaces  at  which  refraction  occurs  are  also  three  in  number :  An- 
terior surface  of  cornea,  radius  of  curvature  8  millimeters;  anterior  surface 
of  lens  radius  of  curvature  10  millimeters;  posterior  surface  of  lens,  radius  of 
curvature  6  millimeters.     It  will  thus  be  seen  that  the  anterior  surface  < 
lens  is  less  and  the  posterior  surface  more  convex  than  the  cornea. 

To  the  values  of  the  optical  constants  of  the  eye  as  above  given  may  I 
added  the  following  :  Distance  from  the  anterior  surface  of  the  cornea  to  the 
anterior  surface  of  the  lens,  3.6  millimeters;  distance  from  the  posterior  sur- 
face of  the  lens  to  the  retina,  15.  millimeters ;  thickness  of  lens,  3.6  millimeters. 
The  methods  usually  employed  for  determining  these  constants  are  the  fol- 
lowing:   The  indices  of  refraction  of  the  aqueous  and  vitreous  humor  are 
determined  by  filling  the  space  between  a  glass  lens  and  a  glass  plate  with  the 
fresh  humor/   The  aqueous  or  vitreous  humor  thus  forms  a  convex  or  concave 
lens  from  the  form  and  focal  distance  of  which  the  index  can  be  calculated. 
Another  method  consists  in  placing  a  thin  layer  of  the  medium  between  the 
hypothenuse  surfaces  of  two  right-angled  prisms  and  determining  the  angle 
which  total  internal  reflection  takes  place.     In  the  case  of  the  crystalline  le 
the  index  is  found  by  determining  its  focal  distance  as  for  an  ordinary  1  ns 
and  solving  the  equation  which  expresses  the  value  of  the  index  in  terms  of 
radius  of  curvature  and  focal  distance,  thickness,  and  focal   length, 
refractive  index  of  the  lens  increases  from  the  surface  toward  the  centre,  a 
peculiarity  which  tends  to  correct  the  disturbances  due  to  spherical  aberration, 
as  well  as  to  increase  the  refractive  power  of  the  lens  as  a  whole. 


750  AN  AMERICAN   TEXT-BOOK  OF  PHYSIOLOGY. 

The  curvature  of  the  refracting  surfaces  of  the  eye  is  determined  by  an 
instrument  known  as  an  ophthalmometer,  which  measures  the  size  of  the 
reflected  image  of  a  known  object  in  the  various  curved  surfaces.  The 
radius  of  curvature  of  the  surface  is  determined  by  the  following  formula : 


r  2Ab 


B  :b  =  A:-;  OT  r  —  — „->  in  which  B  —  the  size  of  the  object,  b  =  the  size  of 

the  image,  A  =  distance  between  the  object  and  the  reflecting  surface,  and 
r  =  the  radius  of  the  reflecting  surface.  The  distances  between  the  various 
surfaces  of  the  eye  are  measured  on  frozen  sections  of  the  organ,  or  can  be 
determined  upon  the  living  eye  by  optical  methods  too  complicated  to  be  here 
described.  It  should  be  borne  in  mind  that  the  above  values  of  the  so-called 
"optical  constants"  of  the  eye  are  subject  to  considerable  individual  variation, 
and  that  the  statements  of  authors  concerning  them  are  not  always  consistent. 
The  refracting  surfaces  of  the  eye  may  be  regarded  as  still  further  sim- 
plified, and  a  so-called  "  reduced  eye "  constructed  which  is  very  useful  for 
purposes  of  optical  calculation.  This  reduced  eye,  which  for  optical  purposes 
is  the  equivalent  of  the  actual  eye,  is  regarded  as  consisting  of  a  single  refract- 
ing medium  having  an  index  of  1.33,  a  radius  of  curvature  of  5.017  milli- 
meters, its  principal  point  2.148  millimeters  behind  the  anterior  surface  of  the 
cornea,  and  its  nodal  point  0.04  millimeter  in  front  of  the  posterior  surface 
of  the  lens.1  The  principal  foci  of  the  reduced  eye  are  respectively  12.918 
millimeters  in  front  of  and  22.231  millimeters  behind  the  anterior  surface  of 
the  cornea.  Its  optical  power  is  equal  to  50.8  dioptrics.2  The  position  of  this 
imaginary  refracting  surface  is  indicated  by  the  dotted  line  in  figure  216.  The 


PIG.  216.— Diagram  of  the  formation  of  a  retinal  image  (after  Foster). 

nodal  point,  n,  in  this  construction  may  be  regarded  as  the  crossing-point  of  all 
the  principal  rays  which  enter  the  eye,  and,  as  these  rays  are  unchanged  in  their 
direction  by  refraction,  it  is  evident  that  the  image  of  the  point  whence  they 
proceed  will  be  formed  at  the  point  where  they  strike  the  retina.  Hence  to 
determine  the  size  and  position  of  the  retinal  image  of  any  external  object — 
e.  g.  the  arrow  in  Figure  216 — it  is  only  necessary  to  draw  lines  from  various 

1  Strictly  speaking,  there  are  in  this  imaginary  refracting  apparatus  which  is  regarded  as 
equivalent  to  the  actual  eye  two  principal  and  two  nodal  points,  each  pair  about  0.4  millimeter 
apart.     The  distance  is  so  small  that  the  two  points  may,  for  all  ordinary  constructions,  be 
regarded  as  coincident. 

2  The  optical  power  of  a  lens  is  the  reciprocal  of  its  focal  length.     The  dioptry  or  unit  of 
optical  power  is  the  power  of  a  lens  with  a  focal  length  of  1  meter. 


THE  SENSE    OF    VISION. 


751 


points  of  the  object  through  the  above-mentioned  nodal  point  and  to  prolong 
them  till  they  strike  the  retina.  It  is  evident  that  the  size  of  the  retinal  image 
will  be  as  much  smaller  than  that  of  the  object  as  the  distance  of  the  nodal 
point  from  the  retina  is  smaller  than  its  distance  from  the  object^ 

According  to  the  figures  above  given,  the  nodal  point  is  about  7.2  milli- 
meters behind  the  anterior  surface  of  the  cornea  and  about  15.0  millimeters  in 
front  of  the  retina.  Hence  the  size  of  the  retinal  image  of  an  object  of  known 
size  and  distance  can  be  readily  calculated — a  problem  which  has  frequently  to  be 
solved  in  the  study  of  physiological  optics.  The  construction  given. in  Figure 
216  shows  that  from  all  external  objects  inverted  images  are  projected  upon  the 
retina,  and  such  inverted  images  can  actually  be  seen  under  favorable  condi- 
tions. If,  for  instance,  the  eye  of  a  white  rabbit,  which  contains  no  choroidal 
pigment,  be  excised  and  held  with  the  cornea  directed  toward  a  window  or 
other  source  of  light,  an  inverted  image  of  the  luminous  object  will  be  seen 
through  the  transparent  sclerotic  in  the  same  way  that  one  sees  an  inverted 
image  of  a  landscape  on  the  ground-glass  plate  of  a  photographic  camera. 
The  question  is  often  asked,  "  Why,  if  the  images  are  inverted  in  the  retina, 
do  we  not  see  objects  upside  down  ?"  The  only  answer  to  such  a  question  is 
that  it  is  precisely  because  images  are  inverted  on  the  retina  that  we  do  not  see 
objects  upside  down,  for  the  eye  has  learned  through  lifelong  practice  to  asso- 
ciate an  impression  made  upon  any  portion  of  the  retina  with  light  coming 
from  the  opposite  portion  of  the  field  of  vision.  Thus  if  an  image  falls  upon 
the  lower  portion  of  the  retina,  our  experience,  gained  chiefly  through  mus- 
cular movements  and  tactile  sensations,  has  taught  us  that  this  image  must  cor- 
respond to  an  object  in  the  upper  portion  of  our  field  of  vision.  In  whatever 
way  the  retina  is  stimulated  the  same  effect  is  produced.  If,  for  instance, 
gentle  pressure  is  made  with  the  finger  on  the  lateral  portion  of  the  eyeball 
through  the  closed  lids  a  circle  of  light  known  as  a  phosphene  immediately 
appears  on  the  opposite  side  of  the  eye.  Another  good  illustration  of  the 
same  general  rule  is  found  in  the  effect  of  throwing  a  shadow  upon  the  retina 
from  an  object  as  close  as  possible  to  the  eye.  For  this  purpose  place  a  card 


B  p 

FIG.  217.— Diagram  illustrating  the  projection  of  a  shadow  on  the  retina. 

with  a  small  pin-hole  in  it  in  front  of  a  source  of  light,  and  three  or  four 
centimeters  distant  from  the  eye.  Then  hold  some  object  smaller  than  the 
pupil — e.  g.  the  head  of  a  pin — as  close  as  possible  to  the  cornea.  Under  these 
conditions  neither  the  pin-hole  nor  the  pin-head  can  be  really  seen — i.  e.  they 


752  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

are  both  too  near  to  have  their  image  focussed  upon  the  retina.  The  pin-hole 
becomes  itself  a  source  of  light,  and  appears  as  a  luminous  circle  bounded  by 
the  shadow  thrown  by  the  edge  of  the  iris.  Within  this  circle  of  light  is  seen 
the  shadow  of  the  pin-head,  but  the  pin-head  appears  inverted,  for  the  obvious 
reason  that  the  eye,  being  accustomed  to  interpret  all  retinal  impressions  as 
corresponding  to  objects  in  the  opposite  portion  of  the  field  of  vision,  regards 
the  upright  shadow  of  the  pin-head  as  the  representation  of  an  inverted  object. 
The  course  of  the  rays  in  this  experiment  is  shown  in  Figure  217,  in  which 
A  R  represents  the  card  with  a  pin-hole  in  it,  P  the  pin,  and  P'  its  upright 
shadow  thrown  on  the  retina. 

Accommodation. — From  what  has  been  said  of  conjugate  foci  and  their 
relation  to  each  other  it  is  evident  that  any  change  in  the  distance  of  the  object 
from  the  refracting  media  will  involve  a  corresponding  change  in  the  position 
of  the  image,  or,  in  other  words,  only  objects  at  a  given  distance  can  be 
focussed  upon  a  plane  which  has  a  fixed  position  with  regard  to  the  refracting 
surface  or  surfaces.  Hence  all  optical  instruments  in  which  the  principle  of 
conjugate  foci  finds  its  application  have  adjustments  for  distance.  In  the 
telescope  and  opera-glass  the  adjustment  is  effected  by  changes  in  the  distance 
between  the  lenses,  and  in  the  photographic  camera  by  a  change  in  the  posi- 
tion of  the  ground-glass  plate  representing  the  focal  plane.  In  the  microscope 
the  adjustment  is  effected  by  changing  the  distance  of  the  object  to  suit  the 
lenses,  the  higher  powers  having  a  shorter  "  working  distance." 

We  must  now  consider  in  what  way  the  eye  adapts  itself  to  see  objects  dis- 
tinctly at  different  distances.  That  this  power  of  adaptation,  or  "  accommo- 
dation," really  exists  we  can  easily  convince  ourselves  by  looking  at  different 
objects  through  a  network  of  fine  wire  held  near  the  eyes.  When  with  normal 
vision  the  eyes  are  directed  to  the  distant  objects  the  network  nearly  disappears, 
and  if  we  attempt  to  see  the  network  distinctly  the  outlines  of  the  distant 
objects  become  obscure.  In  other  words,  it  is  impossible  to  see  both  the 
network  and  the  distant  objects  distinctly  at  the  same  time.  It  is  also  evident 
that  in  accommodation  for  distant  objects  the  eyes  are  at  rest,  for  when  they 
are  suddenly  opened  after  having  been  closed  for  a  short  time  they  are  found 
to  be  accommodated  for  distant  objects,  and  we  are  conscious  of  a  distinct 
effort  in  directing  them  to  any  near  object.1 

From  the  optical  principles  above  described  it  is  clear  that  the  accommo- 
dation of  the  eye  for  near  objects  may  be  conceived  of  as  taking  place  in  three 
different  ways :  1st,  By  an  increase  of  the  distance  between  the  refracting  sur- 
faces of  the  eye  and  the  retina ;  2d,  By  an  increase  of  the  index  of  refraction 
of  one  or  more  of  the  media ;  3d,  By  a  diminution  of  the  radius  of  curvature 
of  one  or  more  of  the  surfaces.  The  first  of  these  methods  was  formerly  sup- 
posed to  be  the  one  actually  in  use,  a  lengthening  of  the  eyeball  under  a  pres- 

1  It  has  been  shown  by  Beer  (Archivfur  die  gesammle  Physiologie,  Iviii.  523)  that  in  fishes 
the  eyes  when  at  rest  are  accommodated  for  near  objects,  and  that  accommodation  for  distant 
objects  is  effected  by  the  contraction  of  a  muscle  for  which  the  name  "retractor  lentis"  is  pro- 
posed. 


THE  SENSE    OF    VISION.  753 

sure  produced  by  the  eye-muscles  being  assumed  to  occur.  This  lengthening 
would,  in  the  case  of  a  normal  eye  accommodating  itself  for  an  object  at  a 
distance  of  15  centimeters,  amount  to  not  less  than  2  millimeters — a  change 
which  could  hardly  be  brought  about  by  the  action  of  any  muscles  connected 
with  the  eye.  Moreover,  accommodation  changes  can  be  observed  upon  elec- 
trical stimulation  of  the  excised  eye.  Its  mechanism  must,  therefore,  lie  within 
the  eye  itself.  As  for  the  second  of  these  methods,  there  is  no  conceivable  way 
by  which  a  change  in  the  index  of  refraction  of  the  media  can  be  eifected,  and 
we  are  thus  forced  to  the  conclusion  that  accommodation  is  brought  about  by 
a  change  in  the  curvature  of  the  refracting  surfaces — i.  e.  by  a  method  quite 
different  from  any  which  is  employed  in  optical  instruments  of  human  con- 
struction. Now,  by  measuring  the  curvature  of  the  cornea  of  a  person  who 
looks  alternately  at  near  and  distant  objects  it  has  been  shown  that  the  cornea 
undergoes  no  change  of  form  in  the  act  of  accommodation.  By  a  process  of 
exclusion,  therefore,  the  lens  is  indicated  as  the  essential  organ  in  this  function 
of  the  eye,  and,  in  fact,  the  complicated  structure  and  connections  of  the  lens 
at  once  suggest  the  thought  that  it  is  in  the  surfaces  of  this  portion  of  the  eye 
that  the  necessary  changes  take  place.  Indeed,  from  a  teleological  point  of 
view  the  lens  would  seem  somewhat  superfluous  if  it  were  not  important  to 
have  a  transparent  refracting  body  of  variable  form  in  the  eye,  for  the  amount 
of  refraction  which  takes  place  in  the  lens  could  be  produced  by  a  slightly 
increased  curvature  of  the  cornea.  Now,  the  changes  of  curvature  which  occur 
in  the  surfaces  of  the  lens  when  the  eye  is  directed  to  distant  and  near  objects 
alternately  can  be  actually  observed  and  measured  with  considerable  accuracy. 
For  this  purpose  the  changes  in  the  form,  size,  and  position  of  the  images  of 
brilliant  objects  reflected  in  these  two  surfaces  are  studied.  If  a  candle  is  held 
in  a  dark  room  on  a  level  with  and  about  50  centimeters  away  from  the  eye  in 
which  the  accommodation  is  to  be  studied,  an  observer,  so  placed  that  his  own 
axis  of  vision  makes  about  the  same  angle  (15°-20°)  with  that  of  the  ob- 
served eye  that  is  made  by  a  line  joining  the  observed  eye  and  the  candle,  will 
readily  see  a  small  upright  image  of  the  candle  reflected  in  the  cornea  of  the 
observed  eye.  Near  this  and  within  the  outline  of  the  pupil  are  two  other 
images  of  the  candle,  which,  though  much  less  easily  seen  than  the  corneal 
image,  can  usually  be  made  out  by  a  proper  adjustment  of  the  light.  The 
first  of  these  is  a  large  faint  upright  image  reflected  from  the  anterior  surface 
of  the  lens,  and  the  second  is  a  small  inverted  image  reflected  from  the  pos- 
terior surface  of  the  lens.  It  will  be  observed  that  the  size  of  these  images 
varies  with  the  radius  of  curvature  of  the  three  reflecting  surfaces  as  given  on 
p.  749.  The  relative  size  and  position  of  these  images  having  been  recog- 
nized while  the  eye  is  at  rest— i.  e.  is  accommodated  for  distance — let  the 
person  who  is  under  observation  be  now  requested  to  direct  his  eye  to  a  near 
object  lying  in  the  same  direction.  When  this  is  done  the  corneal  image  and 
that  reflected  from  the  posterior  surface  of  the  lens  will  remain  unchanged,1 

1  A  very  slight  diminution  in  size  may  sometimes  be  observed  in  the  image  reflected  from 
the  posterior  surface  of  the  lens. 
48 


754 


AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 


while  that  reflected  from  the  anterior  surface  of  the  lens  will  become  smaller 
and  move  toward  the  corneal  image.  This  change  in  the  size  and  position  of 
the  reflected  image  can  only  mean  that  the  surface  from  which  the  reflection 
takes  place  has  become  more  convex  and  has  moved  forward.  Coincident 
with  this  change  a  contraction  of  the  pupil  will  be  observed. 

An  apparatus  for  making  observations  of  this  sort  is  known  as  the  phako- 
scope  of  Helmholtz  (Fig.  218).  The  eye  in  which  the  changes  due  to  accom- 
modation are  to  be  observed  is  placed  at  an  opening 
in  the  back  of  the  instrument  at  C,  and  directed  al- 
ternately to  a  needle  placed  in  the  opening  D  and 
to  a  distant  object  lying  in  the  same  direction.  Two 
prisms  at  B  and  B'  serve  to  throw  the  light  of  a 
candle  on  to  the  observed  eye,  and  the  eye  of  an 
observer  at  A  sees  the  three  reflected  images,  each 
as  two  small  square  spots  of  light.  The  movement 
and  the  change  of  size  of  the  image  reflected  from 
the  anterior  surface  of  the  lens  can  be  thus  much 
better  observed  than  when  a  candle-flame  is  used. 
The  course  of  the  rays  of  light  in  this  experi- 
ment is  shown  diagrammatically  in  Figure  219. 
The  observed  eye  is  directed  to  the  point  A,  while 
the  candle  and  the  eye  of  the  observer  are  placed 
symmetrically  on  either  side.  The  images  of  the  candle  reflected  from  the  various 
surfaces  of  the  eye  will  be  seen  projected  on  the  dark  background  of  the  pupil 


FIG.  218.— Phakoscope  of 
Helmholtz. 


FIG.  219.— Diagram  explaining  the  change  in  the  position  of  the  image  reflected  from  the  anterior  surface 
of  the  crystalline  lens  (Williams,  after  Bonders). 

in  the  directions  indicated  by  the  dotted  lines  ending  at  a,  6,  and  c.  When  the 
eye  is  accommodated  for  a  near  object  the  middle  one  of  the  three  images  moves 
nearer  the  corneal  image — i.  e.  it  changes  in  its  direction  from  b  to  b',  showing 
that  the  anterior  surface  of  the  lens  has  bulged  forward  into  the  position  indi- 


THE   SENSE    OF    VISION. 


755 


cated  by  the  dotted  line.  The  change  in  the  appearance  of  the  images  is 
represented  diagrammatically  in  Figure  220.  On  the  left  is  shown  the  appear- 
ance of  the  images  as  seen  when  the  eye  is  at  rest,  a  representing  the  corneal 
image,  b  that  reflected  from  the  anterior,  and  c  that  from  the  posterior  surface 
of  the  lens  when  the  observing  eye  and  the  candle  are  in  the  position  repre- 


FIG.  220.— Reflected  images  of  a  candle-flame  as  seen  in  the  pupil  of  an  eye  at  rest  and  accommodated 

for  near  objects  (Williams). 

sented  in  Figure  219.  The  images  are  represented  as  they  appear  in  the  dark 
background  of  the  pupil,  though  of  course  the  corneal  image  may,  in  certain 
positions  of  the  light,  appear  outside  of  the  pupillary  region.  When  the  eye 
is  accommodated  for  near  objects  the  images  appear  as  shown  in  the  circle  on 
the  right,  the  image  6  becoming  smaller  and  brighter  and  moving  toward  the 
corneal  image,  while  the  pupil  contracts  as  indicated  by  the  circle  drawn  round 
the  images. 

The  changes  produced  in  the  eye  by  an  effort  of  accommodation  are  indi- 
cated in  Figure  221,  the  left-hand  side  of  the  diagram  showing  the  condition 


FIG.  221.— Showing  changes  in  the  eye  produced  by  the  act  of  accommodation  (Helmholtz). 

of  the  eye  at  rest,  and  the  right-hand  side  that  in  extreme  accommodation  for 
near  objects. 

It  will  be  observed  that  the  iris  is  pushed  forward  by  the  bulging  lens  and 
that  its  free  border  approaches  the  median  line.  In  other  words,  the  pupil  is 
contracted  in  accommodation  for  near  objects.  The  following  explanation  of 
the  mechanism  by  which  this  change  in  the  shape  of  the  lens  is  effected  has 
been  proposed  by  Helmholtz,  and  is  still  generally  accepted.  The  structure 
of  the  lens  is  such  that  by  its  own  elasticity  it  tends  constantly  to  assume  a 
more  convex  form  than  the  pressure  of  the  capsule  and  the  tension  of  the  sus- 
pensory ligaments  (s,  s,  Fig.  221)  allow.  This  pressure  and  tension  are  dimin- 
ished when  the  eye  is  accommodated  for  near  vision  by  the  contraction  of  the 
ciliary  muscles  (c,  c,  Fig.  221),  most  of  whose  fibres,  having  their  origin  at  the 


756  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

point  of  union  of  the  cornea  and  sclerotic,  extend  radially  outward  in  every 
direction  and  are  attached  to  the  front  part  of  the  choroid.  The  contrac- 
tion of  the  ciliary  muscle,  drawing  forward  the  membranes  of  the  eye,  will 
relax  the  tension  of  the  suspensory  ligament  and  allow  the  lens  to  take 
the  form  determined  by  its  own  elastic  structure.  According  to  another 
theory  of  accommodation  proposed  by  Tscherning,1  the  suspensory  liga- 
ment is  stretched  and  not  relaxed  by  the  contraction  of  the  ciliary  muscle. 
In  consequence  of  the  pressure  thus  produced  upon  the 
lens,  the  soft  external  portions  are  moulded  upon  the 
harder  nuclear  portion  in  such  a  way  as  to  give  to  the 
anterior  (and  to  some  extent  to  the  posterior)  surface  a 
hyperboloid  instead  of  a  spherical  form.  A  similar  theory 
has  been  recently  brought  forward  by  Schoen,2  who  com- 
pares the  action  of  the  ciliary  muscle  upon  the  lens  to  that 
of  the  fingers  compressing  a  rubber  ball,  as  shown  in  Fig- 
ure 222.  These  theories  have  an  advantage  over  that 
offered  by  Helmholtz,  inasmuch  as  they  afford  au  expla- 
nation of  the  presence  in  the  ciliary  muscle  of  circular 
fibres,  which,  on  the  theory  of  Helmholtz,  seem  to  be  su- 
perfluous. They  also  make  the  fact  of  so-called  "  astig- 
FIG.  222.— TO  illustrate  matic  accommodation"  comprehensible.  This  term  is 

Schoen's  theory  of  ac-  ,.    ,  ,,  •  i  j.       i  >•  j      n 

commodation.  applied  to  the  power  said  to  be  sometimes  gradually 

acquired  by  persons  with  astigmatic3  eyes  of  correcting 
this  defect  of  vision  by  accommodating  the  eye  more  strongly  in  one  meridian 
than  another.4 

Whatever  views  may  be  entertained  as  to  the  exact  mechanism  by  which  its 
change  of  shape  is  brought  about,  there  can  be  no  doubt  that  the  lens  is  the 
portion  of  the  eye  chiefly  or  wholly  concerned  in  accommodation,  and  it  is 
accordingly  found  that  the  removal  of  the  lens  in  the  operation  for  cataract 
destroys  the  power  of  accommodation,  and  the  patient  is  compelled  to  use 
convex  lenses  for  distant  and  still  stronger  ones  for  near  objects. 

It  is  interesting  to  notice  that  the  act  of  accommodation,  though  distinctly 
voluntary,  is  performed  by  the  agency  of  the  unstriped  fibres  of  the  ciliary 
muscles.  It  is  evident,  therefore,  that  the  term  "  involuntary "  sometimes 
applied  to  muscular  fibres  of  this  sort  may  be  misleading.  The  voluntary 
character  of  the  act  of  accommodation  is  not  affected  by  the  circumstance  that 
the  will  needs,  as  a  rule,  to  be  assisted  by  visual  sensations.  The  fact  that 
most  persons  cannot  affect  the  necessary  change  in  the  eye  unless  they  direct 
their  attention  to  some  near  or  far  object  is  only  an  instance  of  the  close  rela- 
tion between  sensory  impressions  and  motor  impulses,  which  is  further  exem- 

1  Archives  de  Physiologic,  1894,  p.  40.  2  Archiv  fur  die  gesammte  Phys.,  lix.  427. 

3  See  p.  763. 

*  Recent  observations  by  Hess  (Archiv f.  Ophthalmologie,  xlii.  288)  tend  to  confirm  the  Helm- 
holtz theory  by  showing  that  the  suspensory  ligament  is  relaxed  and  not  stretched  in  accommo- 
dation for  near  objects. 


THE  SENSE    OF    VISION.  757 

plified  by  such  phenomena  as  the  paralysis  of  the  lip  of  a  horse  caused  by  the 
division  of  the  trifacial  nerve.  It  is  found,  moreover,  that  by  practice  the 
power  of  accommodating  the  eye  without  directing  it  to  near  and  distant 
objects  can  be  acquired.  The  nerve-channels  through  which_accommodation 
is  affected  are  the  anterior  part  of  the  nucleus  of  the  third  pair  of  nerves 
lying  in  the  extreme  hind  part  of  the  floor  of  the  third  ventricle,  the  most 
anterior  bundle  of  the  nerve-root,  the  third  nerve  itself,  the  lenticular  ganglion, 
and  the  short  ciliary  nerves  (see  diagram  p.  769). 

The  mechanism  of  accommodation  is  affected  in  a  remarkable  way  by  drugs, 
the  most  important  of  which  are  atropia  and  physostigmin,  the  former  para- 
lyzing and  the  latter  stimulating  the  ciliary  muscle.  As  these  drugs  exert  a 
corresponding  effect  upon  the  iris,  it  will  be  convenient  to  discuss  their  action 
in  connection  with  the  physiology  of  that  organ. 

The  changes  occurring  in  the  eye  during  the  act  of  accommodation  are 
indicated  in  the  following  table,  which  shows,  both  for  the  actual  and  the 
reduced  eye,  the  extent  to  which  the  refracting  media  change  their  form  and 
position,  and  the  consequent  changes  in  the  position  of  the  foci  : 

Accommodation  for 
Actual  Eye.  distant  objects.          near  objects. 

Radius  of  cornea 8  mm.  8  mm. 

Radius  of  anterior  surface  of  lens 10  "  6  " 

Radius  of  posterior  surface  of  lens 6  5.5        " 

Distance  from  cornea  to  anterior  surface  of  lens    .    .    3.6  "  3.2        " 

Distance  from  cornea  to  posterior  surface  of  lens      .    7.2  "  7.2        " 

Reduced  Eye. 

Radius  of  curvature 5.02  "                4.48  " 

Distance  from  cornea  to  principal  point 2.15  "                 2.26  " 

Distance  from  cornea  to  nodal  point 7.16  "                 6.74  " 

Distance  from  cornea  to  anterior  focus 12.918  "  11.241  " 

Distance  from  cornea  to  posterior  focus 22.231  "  20.248  " 

It  will  be  noticed  that  no  change  occurs  in  the  curvature  of  the  cornea,  and 
next  to  none  in  the  posterior  surface  of  the  lens,  while  the  anterior  surface  of 
the  lens  undergoes  material  alterations  both  in  its  shape  and  position. 

Associated  with  the  accommodative  movements  above  described,  two  other 
changes  take  place  in  the  eyes  to  adapt  them  for  near  vision.  In  the  first 
place,  the  axes  of  the  eyes  are  converged  upon  the  near  object,  so  that  the 
images  formed  in  the  two  eyes  shall  fall  upon  corresponding  points  of  the 
retinas,  as  will  be  more  fully  explained  in  connection  with  the  subject  of 
binocular  vision.  In  the  second  place,  the  pupil  becomes  contracted,  thus 
reducing  the  size  of  the  pencil  of  rays  that  enters  the  eye.  The  importance 
of  this  movement  of  the  pupil  will  be  better  understood  after  the  subject  of 
spherical  aberration  of  light  has  been  explained.  These  three  adjustments, 
focal,  axial,  and  pupillary,  are  so  habitually  associated  in  looking  at  near  objects 
that  the  axial  can  only  by  an  effort  be  dissociated  from  the  other  two,  while 
these  two  are  quite  inseparable  from  one  another.  This  may  be  illustrated 
by  a  simple  experiment.  On  a  sheet  of  paper  about  40  centimeters  distant 


758  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

from  the  eyes  draw  two  letters  or  figures  precisely  alike  and  about  3  centimeters 
apart.  (Two  letters  cut  from  a  newspaper  and  fastened  to  the  sheet  will  answer 
the  same  purpose.)  Hold  a  small  object  like  the  head  of  a  pin  between  the 
eyes  and  the  paper  at  the  point  of  intersection  of  a  line  joining  the  right  eye 
and  the  left  letter  with  a  line  joining  the  left  eye  and  the  right  letter.  If  the 
axes  of  vision  are  converged  upon  the  pin-head,  that  object  will  be  seen  dis- 
tinctly, and  beyond  it  will  be  seen  indistinctly  three  images  of  the  letter,  the 
central  one  being  formed  by  the  blending  of  the  inner  one  of  each  pair  of 
images  formed  on  the  two  retinas.  If  now  the  attention  be  directed  to  the 
middle  image,  it  will  gradually  become  perfectly  distinct  as  the  eye  accommo- 
dates itself  for  that  distance.  We  have  thus  an  axial  adjustment  for  a  very 
near  object  and  a  focal  adjustment  for  a  more  distant  one.  If  the  pupil  of  the 
individual  making  this  observation  be  watched  by  another  person,  it  will  be 
found  that  at  the  moment  when  the  middle  image  of  the  letter  becomes  distinct 
the  pupil,  which  had  been  contracted  in  viewing  the  pin-head,  suddenly  dilates. 
It  is  thus  seen  that  when  the  axial  and  focal  adjustments  are  dissociated  from 
each  other  the  pupillary  adjustment  allies  itself  with  the  latter. 

The  opposite  form  of  dissociation — viz.  the  axial  adjustment  for  distance 
and  the  focal  adjustment  for  near  vision — is  less  easy  to  bring  about.  It  may 
perhaps  be  best  accomplished  by  holding  a  pair  of  stereoscopic  pictures  before 
the  eyes  and  endeavoring  to  direct  the  right  eye  to  the  right  and  the  left  eye  to 
the  left  picture — i.  e.  to  keep  the  axes  of  vision  parallel  while  the  eyes  are 
accommodated  for  near  objects.  One  who  is  successful  in  this  species  of  ocular 
gymnastics  sees  the  two  pictures  blend  into  one  having  all  the  appearance  of 
a  solid  object.  The  power  of  thus  studying  stereoscopic  pictures  without  a 
stereoscope  is  often  a  great  convenience  to  the  possessor,  but  individuals  differ 
very  much  in  their  ability  to  acquire  it. 

Range  of  Accommodation. — By  means  of  the  mechanism  above  described 
it  is  possible  for  the  eye  to  produce  a  distinct  image  upon  the  retina  of  objects 
lying  at  various  distances  from  the  cornea.  The  point  farthest  from  the  eye 
at  which  an  object  can  be  distinctly  seen  is  called  the  far-point,  and  the  nearest 
point  of  distinct  vision  is  called  the  near-point  of  the  eye,  and  the  distance 
between  the  near-point  and  the  far-point  is  called  the  range  of  distinct  vision 
or  the  range  of  accommodation.  As  the  normal  emmetropic  eye  is  adapted, 
when  at  rest,  to  bring  parallel  rays  of  light  to  a  focus  upon  the  retina,  its  far- 
point  may  be  regarded  as  at  an  infinite  distance.  Its  near-point  varies  with  age, 
as  will  be  described  under  Presbyopia.  In  early  adult  life  it  is  from  10  to 
13  centimeters  from  the  eye.  For  every  point  within  this  range  there  will  be 
theoretically  a  corresponding  condition  of  the  lens  adapted  to  bring  rays  pro- 
ceeding from  that  point  to  a  focus  on  the  retina,  but  as  rays  reaching  the  eye 
from  a  point  175  to  200  centimeters  distant  do  not,  owing  to  the  small  size  of 
the  pupil,  differ  sensibly  from  parallel  rays,  there  is  no  appreciable  change  in 
the  lens  unless  the  object  looked  at  lies  within  that  distance.  It  is  also  evi- 
dent that  as  an  object  approaches  the  eye  a  given  change  of  distance  will 
cause  a  constantly  increasing  amount  of  divergence  of  the  rays  proceeding  from 


THE   SENSE    OF    VISION. 


759 


it,  and  will  therefore  necessitate  a  constantly  increasing  amount  of  change  in 
the  lens  to  enable  it  to  focus  the  rays  on  the  retina.  We  find,  accordingly,  that 
all  objects  more  than  two  meters  distant  from  the  eye  can  be  seen  distinctly  at 
the  same  time — i.  e.  without  any  change  in  the  accommodative  mechanism — 
but  for  objects  within  that  distance  we  are  conscious  of  a  special  effort  of 
accommodation  which  becomes  more  and  more  distinct  the  shorter  the  distance 
between  the  eye  aa<l  the  object. 

Myopia  and  Hypermetropia. — There  are  two  conditions  of  the  eye  in 
which  the  range  of  accommodation  may  differ  from  that  which  has  just  been 
described  as  normal.  These  conditions,  which  are  too  frequent  to  be  regarded 
(except  in  extreme  cases)  as  pathological,  are  generally  dependent  upon  the 
eyeball  being  unduly  lengthened  or 
shortened.  In  Fig.  223  are  shown 
diagram matically  the  three  conditions 
known  as  emmetropia,  myopia,  and 
hypermetropia.  In  the  normal  or 
emmetropic  eye,  A,  parallel  rays  are 
represented  as  brought  to  a  focus  on 
the  retina;  in  the  short-sighted,  or 
myopic,  eye,  B,  similar  rays  are 
focussed  in  front  of  the  retina,  since 
the  latter  is  abnormally  distant;  while 
in  the  over-sighted,  or  hypermetropic, 
eye,  C,  they  are  focussed  behind  the 
retina,  since  it  is  abnormally  near. 

It  is  evident  that  when  the  eye  is 
at  rest  both  the  myopic  and  the  hy- 
permetropic eye  will  see  distant  ob- 
jects indistinctly,  but  there  is  this 
important  difference :  that  in  hyper- 
metropia the  difficulty  can  be  cor- 
rected by  an  effort  of  accommodation, 
while  in  myopia  this  is  impossible, 
since  there  is  no  mechanism  by  which 
the  radius  of  the  lenticular  surfaces  can  be  increased.  Hence  an  individual 
affected  with  myopia  is  always  aware  of  the  infirmity,  while  a  person  with 
hypermetropic  eyes  often  goes  through  life  unconscious  of  the  defect.  In  this 
case  the  accomodation  is  constantly  called  into  play  even  for  distant  objects,  and 
if  the  hypermetropia  is  excessive,  any  prolonged  use  of  the  eyes  is  apt  to  be 
attended  by  a  feeling  of  fatigue,  headache,  and  a  train  of  nervous  symptoms 
familiar  to  the  ophthalmic  surgeon.  Hence  it  is  important  to  discover  this  defect 
where  it  exists  and  to  apply  the  appropriate  remedy — viz.  convex  lenses  placed 
in  front  of  the  eyes  in  order  to  make  the  rays  slightly  convergent  when  they 
enter  the  eye.  Thus  aided,  the  refractive  power  of  the  eye  at  rest  is  sufficient 
to  bring  the  rays  to  a  focus  upon  the  retina  and  thus  relieve  the  accommoda- 


FIG.  223.— Diagram  showing  the  difference  between, 
normal,  myopic,  and  hypermetropic  eyes. 


760  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

tion.  This  action  of  a  convex  lens  in  hyperraetropia  is  indicated  by  the  dotted 
lines  in  Fig.  222,  C,  and  the  corresponding  use  of  a  concave  lens  in  myopia  is 
shown  in  Fig.  222,  B. 

The  detection  and  quantitative  determination  of  hypermetropia  are  best 
made  after  the  accommodation  has  been  paralyzed  by  the  use  of  atropia,  by 
ascertaining  how  strong  a  convex  lens  must  be  placed  before  the  eye  to  pro- 
duce distinct  vision  of  distant  objects. 

The  range  of  accommodation  varies  very  much  from  the  normal  in  myopic 
and  hypermetropic  eyes.  In  myopia  the  near-point  is  often  5  or  6  centimeters 
from  the  cornea,  while  the  far-point,  instead  of  being  infinitely  far  off,  is  at  a 
variable  but  no  very  great  distance  from  the  eye.  The  range  of  accommoda- 
tion is  therefore  very  limited.  In  hypermetropia  the  near-point  is  slightly 
farther  than  normal  from  the  eye,  and  the  far-point  cannot  be  said  to  exist, 
for  the  eye  at  rest  is  adapted  to  bring  converging  rays  to  a  focus  on  the  retina, 
and  such  pencils  of  rays  do  not  exist  in  nature.  Mathematically,  the  far-point 
may  be  said  to  be  at  more  than  an  infinite  distance  from  the  eye.  The  range 
of  effective  accommodation  is  therefore  reduced,  for  a  portion  of  the  accommo- 
dative power  is  used  up  in  adapting  the  eye  to  receive  parallel  rays. 

Presbyopia. — The  power  of  accommodation  diminishes  with  age,  owing 
apparently  to  a  loss  of  elasticity  of  the  lens.  The  change  is  regularly  pro- 
gressive, and  can  be  detected  as  early  as  the  fifteenth  year,  though  in  normal 
eyes  it  does  not  usually  attract  attention  until  the  individual  is  between  forty 
and  forty-five  years  of  age.  At  this  period  of  life  a  difficulty  is  commonly 
experienced  in  reading  ordinary  type  held  at  a  convenient  distance  from  the 
eye,  and  the  individual  becomes  old-sighted  or  presbyopia — a  condition  which 
can,  of  course,  be  remedied  by  the  use  of  convex  glasses.  Cases  are  occasion- 
ally reported  of  persons  recovering  their  power  of  near  vision  in  extreme  old 
age  and  discontinuing  the  use  of  the  glasses  previously  employed  for  reading. 
In  these  cases  there  is  apparently  not  a  restoration  of  the  power  of  accommo- 
dation, but  an  increase  in  the  refractive  power  of  the  lens  through  local  changes 
in  its  tissue.  A  diminution  in  the  size  of  the  pupil,  sometimes  noticed  in  old 
age,  may  also  contribute  to  the  distinctness  of  the  retinal  image,  as  will  be 
described  in  connection  with  spherical  aberration. 

Defects  of  the  Dioptric  Apparatus. — The  above-described  imperfections 
of  the  eye — viz.  myopia  and  hypermetropia — being  generally  (though  not 
invariably)  due  to  an  abnormal  length  of  the  longitudinal  axis,  are  to  be 
regarded  as  defects  of  construction  affecting  only  a  comparatively  small 
number  of  eyes.  There  are,  however,  a  number  of  imperfections  of  the  diop- 
tric apparatus,  many  of  which  affect  all  eyes  alike.  Of  these  imperfections 
some  affect  the  eye  in  common  with  all  optical  instruments,  while  others  are 
peculiar  to  the  eye  and  are  not  found  in  instruments  of  human  construction. 
The  former  class  will  be  first  considered. 

Spherical  Aberration. — It  has  been  stated  that  a  pencil  of  rays  falling 
upon  a  .spherical  refracting  surface  will  be  refracted  to  a  common  focus. 
Strictly  speaking,  however,  the  outer  rays  of  the  pencil — i.  e.  those  which  fall 


THE  SENSE    OF    VISION. 


761 


near  the  periphery  of  the  refracting  surface — will  be  refracted  more  than  those 
which  lie  near  the  axis  and  will  come  to  a  focus  sooner.  This  phenomenon, 
which  is  called  spherical  aberration,  is  more  marked  with  diverging  than  with 
parallel  rays,  and  tends,  of  course,  to  produce  an  indistinctness  of  the  image 
which  will  increase  with  the  extent  of  the  surface  through  which  the  rays 
pass.  The  effect  of  a  diaphragm  used  in  many  optical  instruments  to  reduce 
the  amount  of  spherical  aberration  by  cutting  off  the  side  rays  is  shown  dia- 
grammatically  in  Fig.  224. 


FIG.  224.— Diagram  showing  the  effect  of  a  diaphragm  in  reducing  the  amount  of  spherical 

aberration. 

The  role  of  the  iris  in  the  vision  of  near  objects  is  now  evident,  for  when 
the  eye  is  directed  to  a  near  object  the  spherical  aberration  is  increased  in  con- 
sequence of  the  rays  becoming  more  divergent,  but  the  contraction  of  the 
pupil  which  accompanies  accommodation  tends,  by  cutting  off  the  side  rays,  to 
prevent  a  blurring  of  the  image  which  otherwise  would  be  produced.  It  must, 
however,  be  remembered  that  the  crystalline  lens,  unlike  any  lens  of  human 
construction,  has  a  greater  index  of  refraction  at  the  centre  than  at  the  periph- 
ery. This,  of  course,  tends  to  correct  spherical  aberration,  and,  in  so  far  as  it 
does  so,  to  render  the  cutting  off  of  the  side  rays  unnecessary.  Indeed,  the 
total  amount  of  possible  spherical  aberration  in  the  eye  is  so  small  that  its 
effect  on  vision  may  be  regarded  as  insignificant  in  comparison  with  that  caused 
by  the  other  optical  imperfections  of  the  eye. 

Chromatic  Aberration. — In  the  above  account  of  the  dioptric  apparatus 
of  the  eye  the  phenomena  have  been  described  as  they  would  occur  with  mono- 
chromatic light — i.  e.  with  light  having  but  one  degree  of  refrangibility.  But 
the  light  of  the  sun  is  composed  of  an  infinite  number  of  rays  of  different 
degrees  of  refrangibility.  Hence  when  an  image  is  formed  by  a  simple  lens 
the  more  refrangible  rays — ?'.  e.  the  violet  rays  of  the  spectrum — are  brought 
to  a  focus  sooner  than  the  less  refrangible  red  rays.  The  image  therefore 


762  AX  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 

appears  bordered  by  fringes  of  colored  light.  This  phenomenon  of  chromatic 
aberration  can  be  well  observed  by  looking  at  objects  through  the  lateral  por- 
tion of  a  simple  lens,  or,  still  better,  by  observing  them  through  two  simple 
lenses  held  at  a  distance  apart  equal  to  the  sum  of  their  focal  distances.  The 
objects  will  appear  inverted  (as  through  an  astronomical  telescope)  and  sur- 
rounded with  borders  of  colored  light.  Now,  the  chromatic  aberration  of  the 
eve  is  so  slight  that  it  is  not  easily  detected,  and  the  physicists  of  the  eighteenth 
century,  in  their  efforts  to  produce  an  achromatic  lens,  seem  to  have  been 
impressed  by  the  fact  that  in  the  eye  a  combination  of  media  of  different 
refractive  powers  is  employed,  and  to  have  sought  in  this  circumstance  an 
explanation  of  the  supposed  achromatism  of  the  eye.  Work  directed  on  this 
line  was  crowned  with  brilliant  success,  for  by  combining  two  sorts  of  glass  of 
different  refractive  and  dispersive  powers  it  was  found  possible  to  refract  a  ray 
of  light  without  dispersing  it  into  its  different  colored  rays,  and  the  achromatic 
lens,  thus  constructed,  became  at  once  an  essential  part  of  every  first-class  opti- 
cal instrument.  Now,  as  there  is  not  only  no  evidence  that  the  principle  of 
the  achromatic  lens  is  employed  in  the  eye,  but  distinct  evidence  that  the  eye 
is  uncorrected  for  chromatic  aberration,  we  have  here  a  remarkable  instance  of 
a  misconception  of  a  physical  fact  leading  to  an  important  discovery  in  physics. 
The  chromatic  aberration  of  the  eye,  though  so  slight  as  not  to  interfere  at  all 
with  ordinary  vision,  can  be  readily  shown  to  exist  by  the  simple  experiment 
of  covering  up  one  half  of  the  pupil  and  looking  at  a  bright  source  of  light 
e.  g.  a  window.  If  the  lower  half  of  the  pupil  be  covered,  the  cross-bars  of 


FIG.  225. — Diagram  to  illustrate  chromatic  aberration. 

the  window  will  appear  bordered  with  a  fringe  of  blue  light  on  the  lower  and 
reddish  light  on  the  upper  side.  The  explanation  usually  given  of  the  way  in 
which  this  result  is  produced  is  illustrated  in  Fig.  225.  Owing  to  the  chromatic 
aberration  of  the  eye  all  the  rays  emanating  from  an  object  at  A  are  not 
focussed  accurately  on  the  retina,  but  if  the  eye  is  accommodated  for  a  ray  of 
medium  refrangibility,  the  violet  rays  will  be  brought  to  a  focus  in  front  of 
the  retina  at  I7,  while  the  red  rays  will  be  focussed  behind  the  retina  at  R. 
On  the  retina  itself  will  be  formed  not  an  accurate  optical  image  of  the  point 
A,  but  a  small  circle  of  dispersion  in  which  the  various  colored  rays  are  mixed 
together,  the  violet  rays  after  crossing  falling  upon  the  same  part  of  the  retina 
as  the  red  rays  before  crossing.  Thus  by  a  sort  of  compensation,  which,  how- 
ever, cannot  be  equivalent  tc-  the  synthetic  reproduction  of  white  light  by  the 
union  of  the  spectral  colors,  the  disturbing  effect  of  chromatic  aberration  is 


THE   SENSE    OF    VISION. 


diminished.  When  the  lower  half  of  the  pupil  is  covered  by  the  edge  of  a 
card  held  in  front  of  the  cornea  at  Dy  the  aberration  produced  in  the  upper 
half  of  the  eye  is  not  compensated  by  that  of  the  lower  half.  Hence  the 
image  of  a  point  of  white  light  at  A  will  appear  as  a  row  of  spectral  colors 
on  the  retina,  and  all  objects  will  appear  bordered  by  colored  fringes.  Another 
good  illustration  of  the  chromatic  aberration  of  the  eye  is  obtained  by  cutting 
two  holes  of  any  convenient  shape  in  a  piece  of  black  cardboard  and  placing 
behind  one  of  them  a  piece  of  blue  and  behind  the  other  a  piece  of  red  glass. 
If  the  card  is  placed  in  a  window  some  distance  (10  meters)  from  the  observer, 
iu  such  a  position  that  the  white  light  of  the  sky  may  be  seen  through  the  col- 
ored glasses,  it  will  be  found  that  the  outlines  of  the  two  holes  will  generally 
be  seen  with  unequal  distinctness.  To  most  eyes  the  red  outline  will  appear 
quite  distinct,  while  the  blue  figure  will  seem  much  blurred.  To  a  few  indi- 
viduals the  blue  figure  appears  the  more  distinct,  and  these  will  generally  be 
found  to  be  hypermetropic. 

Astigmatism. — The  defect  known  as  astigmatism  is  due  to  irregularities 
of  curvature  of  the  refracting  surfaces,  in  consequence  of  which  all  the  rays 
proceeding  from  a  single  point  cannot  be  brought  to  a  single  focus  on  the 
retina. 

Astigmatism  is  said  to  be  regular  when  one  of  the  surfaces,  generally  the 
cornea,  is  not  spherical,  but  ellipsoidal — i.  e.  having  meridians  of  maximum 


FIG.  226.— Model  to  illustrate  astigmatism. 


and  minimum  curvature  at  right  angles  to  each  other,  though  in  each  meridians 
the  curvature  is  regular.  When  this  is  the  case  the  rays  proceeding  from  a 
single  luminous  point  are  brought  to  a  focus  earliest  when  they  lie  in  the 
meridian  in  which  the  surface  is  most  convex.  Hence  the  pencil  of  rays  will 


764  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

have  two  linear  foci,  at  right  angles  to  the  meridians  of  greatest  and  least 
curvature  separated  by  a  space  in  which  a  section  of  the  cone  of  rays  will  be 
first  elliptical,  then  circular,  and  then  again  elliptical.  This  defect  exists  to  a 
certain  extent  in  nearly  all  eyes,  and  is,  in  some  cases,  a  serious  obstacle  to  dis- 
tinct vision.  The  course  of  the  rays  when  thus  refracted  is  illustrated  in  Fig.  226, 
which  represents  the  interior  of  a  box  through  which  black  threads  are  drawn 
to  indicate  the  course  of  the  rays  of  light.  The  threads  start  at  one  end  of  the 
box  from  a  circle  representing  the  cornea,  and  converge  with  different  degrees 
of  rapidity  in  different  meridians,  so  that  a  section  of  the  cone  of  rays  will  be 
successively  an  ellipse,  a  straight  line,  an  ellipse,  a  circle,  etc.,  as  shown  by  the 
model  represented  in  Fig.  227.  It  will  be  noticed  that  this  and  the  preced- 


FIG.  227.— Model  to  illustrate  astigmatism. 

ing  figure  are  drawn  in  duplicate,  but  that  the  lines  are  not  precisely  alike  on 
the  two  sides.  In  fact,  the  lines  on  the  left  represent  the  model  as  it  would 
be  seen  with  the  right  eye,  and  those  on  the  right  as  it  would  appear  to 
the  left  eye,  which  is  just  the  opposite  from  an  ordinary  stereoscopic  slide. 
The  figures  are  drawn  in  this  way  because  they  are  intended  to  produce  a 
"  pseudoscopic  "  effect  in  a  way  which  will  be  explained  in  connection  with 
the  subject  of  binocular  vision.  For  this  purpose  it  is  only  necessary  to  cross 
the  axes  of  vision  in  front  of  the  page,  as  in  the  experiment  described  on  page 
758,  for  studying  the  relation  between  the  focal,  axial,  and  pupillary  adjust- 
ments of  the  eye.  As  soon  as  the  middle  image  becomes  distinct  it  assumes  a 
stereoscopic  appearance,  and  the  correct  relations  between  the  different  parts  of 
the  model  are  at  once  obvious. 

This  imperfection  of  the  eye  may  be  detected  by  looking  at  lines  such  as  are 
shown  in  Figure  228,  and  testing  each  eye  separately.     If  the  straight  lines 


THE  SENSE    OF    VISION.  765 

drawn  in  various  directions  through  a  common  point  cannot  be  seen  with  equal 
distinctness  at  the  same  time,  it  is  evident  that  the  eye  is  better  adapted  to  focus 
rays  in  one  meridian  than  in  another — i.  e.  it  is  astigmatic.  The  concentric 


FIG.  228.— Lines  for  the  detection  of  astigmatism. 

circles  are  a  still  more  delicate  test.  Few  persons  can  look  at  this  figure  attentively 
without  noticing  that  the  lines  are  not  everywhere  equally  distinct,  but  that  in 
certain  sectors  the  circles  present  a  blurred  appearance.  Not  infrequently  it 
will  be  found  that  the  blurred  sectors  do  not  occupy  a  constant  position,  but 
oscillate  rapidly  from  one  part  of  the  series  of  circles  to  another.  This  phe- 
nomenon seems  to  be  due  to  slight  involuntary  contractions  of  the  ciliary 
muscle  causing  changes  in  accommodation. 

The  direction  of  the  meridians  of  greatest  and  least  curvature  of  the  cornea 
of  a  regularly  astigmatic  eye,  and  the  difference  in  the  amount  of  this  curvature^ 
can  be  very  accurately  measured  by  means  of  the  ophthalmometer  (see  p.  750). 
These  points  being  determined,  the  defect  of  the  eye  can  be  perfectly  corrected 
by  cylindrical  glasses  adapted  to  compensate  for  the  excessive  or  deficient 
refraction  of  the  eye  in  certain  meridians. 

By  another  method  known  as  "  skiascopy,"  which  consists  in  studying  the 
light  reflected  from  the  fundus  of  the  eye  when  the  ophthalmoscopic  mirror  is 
moved  in  various  directions,  the  amount  and  direction  of  the  astigmatism  of 
the  eye  as  a  whole  (and  not  that  of  the  cornea  alone)  may  be  ascertained. 

Astigmatism  is  said  to  be  irregular  when  in  certain  meridians  the  curvatures 
of  the  refracting  surfaces  are  not  arcs  of  circles  or  ellipses,  or  when  there  is  a 
lack  of  homogeneousness  in  the  refracting  media.  This  imperfection  exists  to 
a  greater  or  less  extent  in  all  eyes,  and,  unlike  regular  astigmatism,  is  incapable 
of  correction.  It  manifests  itself  by  causing  the  outlines  of  all  brilliant  objects 
to  appear  irregular.  It  is  on  this  account  that  the  fixed  stars  do  not  appear  to 
us  like  points  of  light,  but  as  luminous  bodies  with  irregular  "  star  "-shaped 
outlines.  The  phenomenon  can  be  conveniently  studied  by  looking  at  a  pin- 
hole  in  a  large  black  card  held  at  a  convenient  distance  between  the  eye  and  a 
strong  light.  The  hole  will  appear  to  have  an  irregular  outline,  and  to  some 
eyes  will  appear  double  or  treble. 

Intraocular  Images. — Light  entering  the  eye  makes  visible,  under  certain 
circumstances,  a  number  of  objects  which  lie  within  the  eye  itself.  These 
objects  are  usually  opacities  in  the  media  of  the  eye  which  are  ordinarily  invisi- 


766  AN  AMERICAN    TEXT-BOOK   OF  PHYSIOLOGY. 

ble,  because  the  retina  is  illuminated  by  light  coming  from  all  parts  of  the 
pupil,  and  with  such  a  broad  source  of  light  no  object,  unless  it  is  a  very  large 
one  or  one  lying  very  near  the  back  of  the  eye,  can  cast  a  shadow  on  the  retina. 
Such  shadows  can,  however,  be  made  apparent  by  allowing  the  media  of  the 
eye  to  be  traversed  by  parallel  rays  of  light.  This  can  be  accomplished  by 
holding  a  small  polished  sphere — e.  g.  the  steel  head  of  a  shawl-pin  illuminated 
by  sunlight  or  strong  artificial  light — in  the  anterior  focus  of  the  eye — i.  e. 
about  22  millimeters  in  front  of  the  cornea,  or  by  placing  a  dark  screen  with  a 
pin-hole  in  it  in  the  same  position  between  the  eye  and  a  source  of  uniform 
diffused  light,  such  as  the  sky  or  the  porcelain  shade  of  a  student  lamp.  In 
either  case  the  rays  of  light  diverging  from  the  minute  source  will  be  refracted 
into  parallelism  by  the  media  of  the  eye,  and  will  produce  the  sensation  of  a 
circle  of  diffused  light,  the  size  of  which  will  depend  upon  the  amount  of  dila- 
tation of  the  pupil.  Within  this  circle  of  light  will  be  seen  the  shadows  of  any 
opaque  substances  that  may  be  present  in  the  media  of  the  eye.  These  shadows, 
being  cast  by  parallel  rays,  will  be  of  the  same  size  as  the  objects  themselves, 
as  is  shown  diagram matically  in  Figure  229,  in  which  A  represents  a  source 


FIG.  229.— Showing  the  method  of  studying  intraocular  images  (Helmholtz). 

of  light  at  the  anterior  focus  of  the  eye,  and  b  an  opacity  in  the  vitreous  humor 
casting  a  shadow  B  of  the  same  size  as  itself  upon  the  retina.  It  is  evident  that 
if  the  source  of  light  A  is  moved  from  side  to  side  the  various  opacities  will  be 
displaced  relatively  to  the  circle  of  light  surrounding  them  by  an  amount  de- 
pending upon  the  distance  of  the  opacities  from  the  retina.  A  study  of  these 
displacements  will  therefore  afford  a  means  of  determining  the  position  of  the 
opacities  within  the  media  of  the  eye. 

Muscae  Volitantes. — Among  the  objects  to  be  seen  in  thus  examining  the 
eye  the  most  conspicuous  are  those  known  as  the  muscce  volitantes.  These  pre- 
sent themselves  in  the  form  of  beads,  either  singly  or  in  groups,  or  of  streaks, 
patches,  and  granules.  They  have  an  almost  constant  floating  motion,  which 
is  increased  by  the  movements  of  the  eye  and  head.  They  usually  avoid  the 
line  of  vision,  floating  away  when  an  attempt  is  made  to  fix  the  sight  upon 
them.  When  the  eye  is  directed  vertically,  however,  they  sometimes  place 
themselves  directly  in  line  with  the  object  looked  at.  If  the  intraocular  object 
is  at  the  same  time  sufficiently  near  the  back  of  the  eye  to  cast  a  shadow  which 
is  visible  without  the  use  of  the  focal  illumination,  some  inconvenience  may 
thus  be  caused  in  using  a  vertical  microscope. 

A  study  of  the  motions  of  the  muscce  volitantes  makes  it  evident  that  the 


THE  SENSE   OF    VISION.  767 

phenomenon  is  due  to  small  bodies  floating  in  a  liquid  medium  of  a  little 
greater  specific  gravity  than  themselves.  Their  movements  are  chiefly  in 
planes  perpendicular  to  the  axis  of  vision,  for  when  the  eye  is  directed  verti- 
cally upward  they  move  as  usual  through  the  field  of  vision  without  increasing 
the  distance  from  the  retina.  They  are  generally  supposed  to  be  the  remains 
of  the  embyronic  structure  of  the  vitreous  body — i.  e.  portions  of  the  cells  and 
fibres  which  have  not  undergone  complete  mucous  transformation. 

In  addition  to  these  floating  opacities  in  the  vitreous  body  various  other 
defects  in  the  transparent  media  of  the  eye  may  be  revealed  by  the  method  of 
focal  illumination.  Among  these  may  be  mentioned  spots  and  stripes  due  to 
irregularities  in  the  lens  or  its  capsule,  and  radiating  lines  indicating  the  stel- 
late structure  of  the  lens. 

Retinal  Vessels. — Owing  to  the  fact  that  the  blood-vessels  ramify  near  the 
anterior  surface  of  the  retina,  while  those  structures  which  are  sensitive  to  light 
constitute  the  posterior  layer  of  that  organ,  it  is  evident  that  light  entering  the 
eye  will  cast  a  shadow  of  the  vessels  on  the  light-perceiving  elements  of  the 
retina.  Since,  however,  the  diameter  of  the  largest  blood-vessels  is  not  more 
than  one-sixth  of  the  thickness  of  the  retina,  and  the  diameter  of  the  pupil  is 
one-fourth  or  one-fifth  of  the  distance  from  the  iris  to  the  retina,  it  is  evident 
that  when  the  eye  is  directed  to  the  sky  or  other  broad  illuminated  surfaces  it 
is  only  the  penumbra  of  the  vessels  that  will  reach  the  rods  and  cones,  the  umbra 
terminating  conically  somewhere  in  the  thickness  of  the  retina.  But  if  light 
is  allowed  to  enter  the  eye  through  a  pin-hole  in  a  card  held  a  short  distance 
from  the  cornea,  as  in  the  above-described  method  of  focal  illumination,  a 
sharply  defined  shadow  of  the  vessels  will  be  thrown  on  the  rods  and  cones. 
Yet  under  these  conditions  the  retinal  vessels  are  not  rendered  visible  unless 
the  perforated  card  is  moved  rapidly  to  and  fro,  so  as  to  throw  the  shadow 
continually  on  to  fresh  portions  of  the  retinal  surface.  When  this  is  done  the 
vessels  appear,  ramifying  usually  as  dark  lines  on  a  lighter  background,  but 
the  dark  lines  are  sometimes  bordered  by  bright  edges.  It  will  be  observed 
that  those  vessels  appear  most  distinctly  the  course  of  which  is  at  right  angles 
to  the  direction  in  which  the  card  is  moved.  Hence  in  order  to  see  all  the 
vessels  with  equal  distinctness  it  is  best  to  move  the  card  rapidly  in  a  circle 
the  diameter  of  which  should  not  exceed  that  of  the  pupil.  In  this  manner 
the  distribution  of  the  vessels  in  one's  own  retina  may  be  accurately  observed, 
and  in  many  cases  the  position  of  the  fovea  centralis  may  be  determined  by  the 
absence  of  vessels  from  that  portion  of  the  macula  lutea. 

The  retinal  vessels  may  also  be  made  visible  in  several  other  ways — e.  g., 
1.  By  directing  the  eye  toward  a  dark  background  and  moving  a  candle  to  and 
fro  in  front  of  the  eye,  but  below  or  to  one  side  of  .the  line  of  vision.  2.  By 
concentrating  a  strong  light  by  means  of  a  lens  of  short  focus  upon  a  point 
of  the  sclerotic  as  distant  as  possible  from  the  cornea.  By  either  of  these 
methods  a  small  image  of  the  external  source  of  light  is  formed  upon  the 
lateral  portion  of  the  eye,  and  this  image  is  the  source  of  light  which  throws 
shadows  of  the  retinal  vessels  on  to  the  rods  and  cones. 


768  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

Circulation  of  Blood  in  the  Retina. — When  the  eye  is  directed  toward  a 
surface  which  is  uniformly  and  brightly  illuminated — e.  g.  the  sky  or  a  sheet 
of  white  paper  on  which  the  sun  is  shining — the  field  of  vision  is  soon  seen  to 
be  filled  with  small  bright  bodies  moving  with  considerable  rapidity  in  irregu- 
lar curved  lines,  but  with  a  certain  uniformity  which  suggests  that  their 
movements  are  confined  to  definite  channels.  They  are  usually  better  seen 
when  one  or  more  sheets  of  cobalt  glass  are  held  before  the  face,  so  that  the 
eyes  are  bathed  in  blue  light.  That  the  phenomenon  depends  upon  the  circu- 
lation of  the  blood  globules  in  the  retina  is  evident  from  the  fact  that  the 
moving  bodies  follow  paths  which  correspond  with  the  form  of  the  retinal 
capillaries  as  seen  by  the  methods  above  described,  and  also  from  the  corre- 
spondence between  the  rate  of  movement  of  the  intraocular  image  and  the 
rapidity  of  the  capillary  circulation  in  those  organs  in  which  it  can  be  di- 
rectly measured  under  the  microscope.  The  exact  way  in  which  the  moving 

\  globules  stimulate  the  retina  so  as  to  produce  the  observed  phenomenon  must 

j  be  regarded  as  an  unsettled  question. 

We  have  thus  seen  that  the  eye,  regarded  from  the  optician's  point  of  view, 
has  not  only  all  the  faults  inherent  in  optical  instruments  generally,  but  many 
others  which  would  not  be  tolerated  in  an  instrument  of  human  construction. 
Yet  with  all  its  imperfections  the  eye  is  perhaps  the  most  wonderful  instance 
in  nature  of  the  development  of  a  highly  specialized  organ  to  fulfil  a  definite 

\  purpose.     In  the  accomplishment  of  this  object  the  various  parts  of  the  eye 

*have  been  perfected  to  a  degree  sufficient  to  enable  it  to  meet  the  requirements 
of  the  nervous  system  with  which  it  is  connected,  and  no  farther.  In  the 
ordinary  use  of  the  eye  we  are  unconscious  of  its  various  irregularities,  shadows, 
opacities,  etc.,  for  these  imperfections  are  all  so  slight  that  the  resulting  inac- 
curacy of  the  image  does  not  much  exceed  the  limit  which  the  size  of  the 
light-perceiving  elements  of  the  retina  imposes  upon  the  delicacy  of  our  visual 
perceptions,  and  it  is  only  by  illuminating  the  eye  in  some  unusual  way  that 
the  existence  of  these  imperfections  can  be  detected.  In  other  words,  the  eye 
is  as  good  an  optical  instrument  as  the  nervous  system  can  appreciate  and 
make  use  of.  Moreover,  when  we  reflect  upon  the  difficulty  of  the  problem 
which  nature  has  solved,  of  constructing  an  optical  instrument  out  of  living 
and  growing  animal  tissue,  we  cannot  fail  to  be  struck  by  the  perfection  of  the 
dioptric  apparatus  of  the  eye  as  well  as  by  its  adaptation  to  the  needs  of  the 
organism  of  which  it  forms  a  part. 

Iris. — The  importance  of  the  iris  as  an  adjustable  diaphragm  for  cutting 
off  side  rays  and  thus  securing  good  definition  in  near  vision  has  been  described 
in  connection  with  the  act  of  accommodation.  Its  other  function  of  protecting 
the  retina  from  an  excess  of  light  is  no  less  important,  and  we  must  now  con- 
sider how  this  pupillary  adjustment  may  be  studied  and  by  what  mechanism 
it  is  effected.  The  changes  in  the  size  of  the  pupil  may  be  conveniently  ob- 
served in  man  and  animals  by  holding  a  millimeter  scale  in  front  of  the  eye 
and  noticing  the  variations  in  the  diameter  of  the  pupil.  It  should  be  borne 
in  mind  that  the  iris,  seen  in  this  way,  does  not  appear  in  its  natural  size  and 


THE  SENSE    OF    VISION. 


769 


position,  but  somewhat  enlarged  and  bulged  forward  by  the  magnifying  effect 
of  the  cornea  and  the  aqueous  humor.  The  changes  in  one's  own  pupil  may 
be  readily  observed  by  noticing  the  varying  size  of  the  circle  of  light  thrown 
upon  the  retina  when  the  eye  is  illuminated  by  a  point  of  light  held  at  the 
anterior  focus,  as  in  the  method  above  described  for  the  study  of  intraocular 
images. 

The  muscles  of  the  iris  are,  except  in  birds,  of  the  unstriped  variety,  and 
are  arranged  concentrically  around  the  pupil.  Radiating  fibres  are  also  recog- 
nized by  many  observers,  though  their  existence  has  been  called  in  question 
by  others.  The  circular  or  constricting  muscles  of  the  iris  are  under  the  con- 
trol of  the  third  pair  of  cranial  nerves, 
and  are  normally  brought  into  activity 
in  consequence  of  light  falling  upon 
the  retina.  This  is  a  reflex  phenom- 
enon, the  optic  nerve  being  the  affer- 
ent, and  the  third  pair,  the  ciliary 
ganglion,  and  the  short  ciliary  nerves 
the  efferent,  channel,  as  indicated  in 
Figure  230.  This  reflex  is  in  man 
and  many  of  the  higher  animals  bi- 
lateral— i.  e.  light  falling  upon  one 
retina  will  cause  a  contraction  of  both 
pupils.  This  may  readily  be  observed 
in  one's  own  eye  when  focally  illumi- 
nated in  the  manner  above  described. 
Opening  the  other  eye  will,  under 
these  conditions,  cause  a  diminution, 
and  closing  it  an  increase,  in  the  size 
of  the  circle  of  light.  This  bilateral 
character  is  found  to  be  dependent 
upon  the  nature  of  the  decussation  of 
the  optic  nerves,  for  in  animals  in 
which  the  crossing  is  complete  the 

reflex    is   confined    to    the    illuminated     nerves  governing  the  pupil  (after  Foster) :  II,  optic 
rpi  /»     i        ft  i  nerve ;  1.  g,  ciliary  ganglion ;  r.  b,  its  short  root  from 

eye.      Ine  arrangement  ot  the  nbres    ///,  motor-ocuii  nerve  -.sym,  its  sympathetic  root  -,r.i, 

in    the    Optic   commissure  is  in  general    its  long  root  from  F,ophthalmo-nasal  branch  of  oph- 
T         .  ,        ,  .  .  ft      t        thalmic  division  of  fifth  nerve ;  8.  c.  short  ciliary 

associated    With    the    position    of     the    nerves ;  1.  c,  long  ciliary  nerves. 

eyes   in   the   head.     When    the   eyes 

are  so  placed  that  they  can  both  be  directed  to  the  same  object,  as  in  man 
and  many  of  the  higher  animals,  the  fibres  of  each  optic  nerve  are  usually 
found  to  be  distributed  to  both  optic  tracts,  while  in  animals  whose  eyes 
are  in  opposite  sides  of  the  head  there  is  complete  crossing  of  the  optic  nerves. 
Hence  it  may  be  said  that  animals  having  binocular  vision  have  in  general 
a  bilateral  pupillary  reflex.  The  rule  is,  however,  not  without  exceptions, 
for  owls,  though  their  visual  axes  are  parallel,  have,  like  other  birds,  a  corn- 


course  of  constrictor  nerve-fibres  •- 
"        dilator  " 


FIG.  230.— Diagrammatic   representation  of  the 


770  AN  AMERICAN  TEXT-BOOK   OF  PHYSIOLOGY. 

plete  crossing  of  the  optic  nerves,  and  consequently  a  unilateral  pupillary 
reflex.1 

A  direct  as  well  as  a  reflex  constriction  of  the  pupil  under  the  influence  of 
light  has  been  observed  in  the  excised  eyes  of  eels,  frogs,  and  some  other  ani- 
mals. As  the  phenomenon  can  be  seen  in  preparations  consisting  of  the  iris 
alone  or  of  the  iris  and  cornea  together,  it  is  evident  that  the  light  exerts  its 
influence  directly  upon  the  tissues  of  the  iris  and  not  through  an  intraocular 
connection  with  the  retina.  The  maximum  effect  is  produced  by  the  yellowish- 
green  portion  of  the  spectrum. 

Antagonizing  the  motor  oculi  nerve  in  its  constricting  influence  on  the 
pupil  is  a  set  of  nerve-fibres  the  function  of  which  is  to  increase  the  size  of 
the  pupil.  Most  of  these  fibres  seem  to  run  their  course  from  a  centre  which 
lies  in  the  floor  of  the  third  ventricle  not  far  from  the  origin  of  the  third  pair, 
through  the  bulb,  the  cervical  cord,  the  anterior  roots  of  the  upper  dorsal 
nerves,  the  upper  thoracic  ganglion,  the  cervical  sympathetic  nerve  as  far  as 
the  upper  cervical  ganglion ;  then  through  a  branch  which  accompanies  the 
internal  carotid  artery,  passes  over  the  Gasserian  ganglion  and  joins  the  oph- 
thalmic branch  of  the  fifth  pair ;  then  through  the  nasal  branch  of  the  latter 
nerve  and  the  long  ciliary  nerves  to  the  eye 2  (see  diagram,  p.  769).  These 
fibres  appear  to  be  in  a  state  of  tonic  activity,  for  section  of  them  in  any  part 
of  their  course  (most  conveniently  in  the  cervical  sympathetic)  causes  a  con- 
traction of  the  pupil  which,  on  stimulation  of  the  peripheral  end  of  the  divided 
nerve,  gives  place  to  a  marked  dilatation.  Their  activity  can  be  increased  in 
various  ways.  Thus  dilatation  of  the  pupil  may  be  caused  by  dyspnea,  vio- 
lent muscular  efforts,  etc.  Stimulation  of  various  sensory  nerves  may  also* 
cause  reflex  dilatation  of  the  pupil,  and  this  phenomenon  may  be  observed, 
though  greatly  diminished  in  intensity,  after  extirpation  of  the  superior  cervi- 
cal sympathetic  ganglion.  It  is  therefore  evident  that  the  dilator  nerves  of  the 
pupil  do  not  have  their  course  exclusively  in  the  cervical  sympathetic  nerve. 

Since  the  cervical  sympathetic  nerve  contains  vaso-constrictor  fibres  for  the 
head  and  neck,  it  has  been  thought  that  its  dilating  effect  upon  the  pupil  might 
be  explained  by  its  power  of  causing  changes  in  the  amount  of  blood  in  the 
vessels  of  the  iris.  There  is  no  doubt  that  a  condition  of  vascular  turgescence 
or  depletion  will  tend  to  produce  contraction  or  dilatation  of  the  pupil,  but  it  is 
impossible  to  explain  the  observed  phenomena  in  this  way,  since  the  pupillary 
are  more  prompt  than  the  vascular  changes,  and  may  be  observed  on  a  bloodless 
eye.  Moreover,  the  nerve-fibres  producing  them  are  said  to  have  a  somewhat 
different  course.  Another  explanation  of  the  influence  of  the  sympathetic  on 
the  pupil  is  that  it  acts  by  inhibiting  the  contraction  of  the  sphincter  muscles,, 
and  that  the  dilatation  is  simply  an  elastic  reaction.  But  since  it  is  posssible  to 
produce  local  dilatation  of  the  pupil  by  circumscribed  stimulation  at  or  near 

1  Steinach  :  Archiv  fur  die  gesammte  Physiologic,  xlvii.  313. 

'2  Langley  :  Journal  of  Physiology,  xiii.  p.  575.  For  the  evidence  of  the  existence  of  a 
"cilio-spinal"  centre  in  the  cord,  see  Steil  and  Langendorff:  Archiv  fur  die  gesammte  Phys- 
ioiogif,  Iviii.  p.  155 ;  also  Schenck :  Ibid.,  Ixii.  p.  494. 


THE   SENSE    OF    VISION.  Ill 

the  outer  border  of  the  iris,  it  seems  more  reasonable  to  conclude  that  the 
dilator  nerves  of  the  pupil  act  upon  radial  muscular  fibres  in  the  substance  of 
the  iris,  in  spite  of  the  fact  that  the  existence  of  such  fibres  has  not  been  uni- 
versally admitted. 

Whatever  view  may  be  taken  of  the  mechanism  by  which  the  sympathetic 
nerves  influence  the  pupil,  there  is  no  doubt  that  the  iris  is  under  the  control 
of  two  antagonistic  sets  of  nerve-fibres,  both  of  which  are,  under  normal  cir- 
cumstances, in  a  state  of  tonic  activity.  Therefore,  when  the  sympathetic 
nerve  is  divided  the  pupil  contracts  under  the  influence  of  the  motor  oculi,  and 
section  of  the  motor  oculi  causes  dilatation  through  the  unopposed  influence  of 
the  sympathetic. 

The  movements  of  the  iris,  though  performed  by  smooth  muscles,  are  more 
rapid  than  those  of  smooth  muscles  found  elsewhere — e.  g.  in  'the  intestines 
and  the  arteries.  The  contraction  of  the  pupil  when  the  retina  of  the  oppo- 
site eye  is  illuminated  occupies  about  0.3' f ;  the  dilatation  when  the  light  is  cut 
off  from  the  eye,  about  3"  or  4".  The  latter  determination  is,  however,  diffi- 
cult to  make  with  precision,  since  dilatation  of  the  pupil  takes  place  at  first 
rapidly  and  then  more  slowly,  so  that  the  moment  when  the  process  is  at  an 
end  is  not  easily  determined.  After  remaining  a  considerable  time  in  absolute 
darkness  the  pupils  become  enormously  dilated,  as  has  been  shown  by  flash- 
light photographs  taken  under  these  conditions.  In  sleep,  though  the  eyes  are 
protected  from  the  light,  the  pupils  are  strongly  contracted,  but  dilate  on 
stimulation  of  sensory  nerves,  even  though  the  stimulation  may  be  insufficient 
to  rouse  the  sleeper. 

Many  drugs  when  introduced  into  the  system  or  applied  locally  to  the  con- 
junctiva produce  effects  upon  the  pupil.  Those  which  dilate  it  are  known  as 
mydriatics,  those  which  contract  it  as  myotics.  Of  the  former  class  the  most 
important  is  atropin,  the  alkaloid  of  the  Atropa  belladonna,  and  of  the  latter 
physostigmin,  the  alkaloid  of  the  Calabar  bean.  In  addition  to  their  action 
upon  the  pupil,  mydriatics  paralyze  the  accommodation,  thus  focussing  the  eye 
for  distant  objects,  while  myotics,  by  producing  a  cramp  of  the  ciliary  muscle, 
adjust  the  eye  for  near  vision.  The  effect  on  the  accommodation  usually 
begins  later  and  passes  off  sooner  than  the  affection  of  the  pupil.  Atropin 
seems  to  act  by  producing  local  paralysis  of  the  terminations  of  the  third  pair 
of  cranial  nerves  in  the  sphincter  iridis  and  the  ciliary  muscle.  In  large 
doses  it  may  also  paralyze  the  muscle-fibres  of  the  sphincter.  With  this  para- 
lyzing action  there  appears  to  be  combined  a  stimulating  effect  upon  the  dilator 
muscles  of  the  iris.  The  myotic  action  of  physostigmin  seems  to  be  due  to  a 
local  stimulation  of  the  fibres  of  the  sphincter  of  the  iris. 

Although  in  going  from  a  dark  room  to  a  lighter  one  the  pupil  at  first  con- 
tracts, this  contraction  soon  gives  place  to  a  dilatation,  and  in  about  three  or 
four  minutes  the  pupil  usually  regains  its  former  size.  In  a  similar  manner 
the  primary  dilatation  of  the  pupil  caused  by  entering  a  dark  room  from  a 
lighter  one  is  followed  by  a  contraction  which  usually  restores  the  pupil  to  its 
original  size  within  fifteen  or  twenty  minutes.  It  is  thus  evident  that  the 


772 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


amount  of  light  falling  upon  the  retina  is  not  the  only  factor  in  determining 
the  size  of  the  pupil.  In  fact,  if  the  light  acts  for  a  sufficient  length  of  time 
the  pupil  may  have  the  same  size  under  the  influence  of  widely  different 
degrees  of  illumination.1 

This  so-called  "  adaptation  "  of  the  eye  to  various  amounts  of  light  seems 
to  be  connected  with  the  movements  of  the  retinal  pigment-granules  and  with 
the  chemical  changes  of  the  visual  purple,  to  be  more  fully  described  in  con- 
nection with  the  physiology  of  the  retina. 

The  Ophthalmoscope. — Under  normal  conditions  the  pupil  of  the  eye 
appears  as  a  black  spot  in  the  middle  of  the  colored  iris.  The  cause  of  this 
dark  appearance  of  the  pupil  is  to  be  found  in  the  fact  that  a  source  of  light 
and  the  retina  lie  in  the  conjugate  foci  of  the  dioptric  apparatus  of  the  eye. 
Hence  any  light  entering  the  eye  that  escapes  absorption  by  the  retinal  pig- 
ment and  is  reflected  from  the  fundus  must  be  refracted  back  to  the  source 
from  which  it  came.  The  eye  of  an  observer  who  looks  at  the  pupil  from 
another  direction  will  see  no  light  coming  from  it,  and  it  will  therefore  appear 
to  him  black.  It  is  therefore  evident  that  the  essential  condition  for  perceiving 
light  coming  from  the  fundus  of  the  eye  is  that  the  line  of  vision  of  the 
observing  eye  shall  be  in  the  line  of  illumination.  This  condition  is  fulfilled 
by  means  of  instruments  known  as  ophthalmoscopes.  The  principles  involved 
in  the  construction  of  the  most  common  form  of  ophthalmoscope  are  illustrated 
diagrammatically  in  Figure  231. 


FIG.  231.— Diagram  to  illustrate  the  principles  of  a  simple  ophthalmoscope  (after  Foster). 

The  rays  from  a  source  of  light  Z,  after  being  brought  to  a  focus  at  a  by 
the  concave  perforated  mirror  M  M,  pass  on  and  are  rendered  parallel  by  the 
lens  I.  Then,  entering  the  observed  eye  5,  they  are  brought  to  a  focus  on  the 
retina  at  a'.  Any  rays  which  are  reflected  back  from  the  part  of  the  retina 
thus  illuminated  will  follow  the  course  of  the  entering  rays  and  be  brought  to 
a  focus  at  a.  The  eye  of  an  observer  at  A,  looking  through  the  hole  in  the 
mirror,  will  therefore  see  at  a  an  inverted  image  of  the  retina,  the  observation 
of  which  may  be  facilitated  by  a  convex  lens  placed  immediately  in  front  of 
the  observer's  eye. 

1  Schirmer  :  Archivfur  Ophthalmologie,  xi.  5. 


THE  SENSE    OF    VISION.  773 

The  fund  us  of  the  eye  thus  observed  presents  a  reddish  background  on 
which  the  retinal  vessels  are  distinctly  visible. 

Retina. — Having  considered  the  mechanism  by  which  optical  images  of 
objects  at  various  distances  from  the  eye  are  formed  upon  the  retina,  we  must 
next  inquire  what  part  of  the  retina  is  affected  by  the  rays  of  light,  and  in 
what  this  affection  consists.  To  the  former  of  these  questions  it  will  be  found 
possible  to  give  a  fairly  satisfactory  answer,  With  regard  to  the  latter  nothing 
positive  is  known. 

The  structure  of  the  retina  is  exceedingly  complicated,  but,  as  very  little 
is  known  of  the  functions  of  the  ganglion  cells  and  of  the  molecular  and 
nuclear  layers,  it  will  suffice  for  the  present  purpose  of  physiological  descrip- 
tion to  regard  the  retina  as  consisting  of  fibres  of  the  optic  nerve  which  are 
connected  through  various  intermediate  structures  with  the  layer  of  rods  and 
cones. 


A 
FIG.  232.— Diagrammatic  representation  of  the  retina. 

Figure  232  is  intended  to  show,  diagram  matically,  the  mutual  relation  of 
these  various  portions  of  the  retina  in  different  parts  of  the  eye,  and  is  not 
drawn  to  scale.  It  will  be  observed  that  the  optic  nerve  0,  where  it  enters  the 
eye,  interrupts  the  continuity  of  the  layer  of  rods  and  cones  R  and  of  the 
intermediate  structures  /.  Its  fibres  spread  themselves  out  in  all  directions, 
forming  the  internal  layer  of  the  retina  N.  The  central  artery  of  the  retina 
A  accompanying  the  optic  nerve  ramifies  in  the  layer  of  nerve-fibres  and  in 
the  immediately  adjacent  layers  of  the  retina,  forming  a  vascular  layer  V.  In 
the  fovea  centralis  F  of  the  macula  lutea  (the  centre  of  distinct  vision)  the 
layer  of  rods  and  cones  becomes  more  highly  developed,  while  the  other  layers 
of  the  retina  are  much  reduced  in  thickness  and  the  blood-vessels  entirely  dis- 
appear. This  histological  observation  points  strongly  to  the  conclusion  that 
the  rods  and  cones  are  the  structures  which  are  essential  to  vision,  and  that  in 
them  are  found  the  conditions  for  the  conversion  of  the  vibrations  of  the 
luminiferous  ether  into  a  stimulus  for  a  nerve-fibre.  This  view  derives  con- 
firmation from  the  observations  on  the  retinal  blood-vessels,  for  it  is  found 
that  the  distance  between  the  vascular  layer  of  the  retina  and  the  layer 
of  rods  and  cones  determined  by  histological  methods  corresponds  with  that 
which  must  exist  between  the  vessels  and  the  light-perceiving  elements  of  the 
retina,  as  calculated  from  the  apparent  displacement  of  the  shadow  caused  by 
given  movements  of  the  source  of  light  used  in  studying  intraocular  images l  as 

1  "  Dimmer  Verh.  d.  phys.  Clubs  zu  Wien,  24  April,  1894,"  Ceniralbl.fur  Physiologic,  1894, 159. 


774 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


described  on  p.  767.  Another  argument  in  favor  of  this  view  is  found  in  the 
correspondence  between  the  size  of  the  smallest  visible  images  on  the  retina  and 
the  diameter  of  the  rods  and  cones.  A  double  star  can  be  recognized  as  double 
by  the  normal  eye  when  the  distance  between  the  components  corresponds  to 
a  visual  angle  of  60".  Two  white  lines  on  a  black  ground  are  seen  to  be  dis- 
tinct when  the  distance  between  them  subtends  a  visual  angle  of  64r/-73r/. 
These  angles  correspond  to  a  retinal  image  of  0.0044,  0.0046,  and  0.0053  mil- 
limeter. Now,  the  diameter  of  the  cones  in  the  macula  lutea,  as  determined 
by  Kolliker,  is  0.0045-0.0055  millimeter,  a  size  which  agrees  well  with  the 
hypothesis  that  each  cone  when  stimulated  can  produce  a  special  sensation  of 
light  distinguishable  from  those  caused  by  the  stimulation  of  the  neighboring 
cones.  The  existence  of  the  so-called  blind  spot  in  the  retina  at  the  point  of 
entrance  of  the  optic  nerve  is  sometimes  regarded  as  evidence  of  the  light- 
perceiving  function  of  the  rods  and  cones,  but  as  the  other  layers  of  the  retina, 
as  well  as  the  rods  and  cones,  are  absent  at  this  point,  and  the  retina  here 
consists  solely  of  nerve-fibres,  it  is  evident  that  the  presence  of  the  blind  spot 


FIG.  233.— To  demonstrate  the  blind  spot. 

only  proves  that  the  optic  nerve-fibres  are  insensible  to  light.  Figure  233  is 
intended  to  demonstrate  this  insensibility.  For  this  purpose  it  should  be  held 
at  a  distance  of  about  23  centimeters  from  the  eyes  (i.  e.  about  3.5  times  the  dis- 
tance between  the  cross  and  the  round  spot).  If  the  left  eye  be  closed  and  the 
right  eye  fixed  upon  the  cross,  the  round  spot  will  disappear  from  view,  though 
it  will  become  visible  if  the  eye  be  directed  either  to  the  right  or  to  the  left  of 
the  cross,  or  if  the  figure  be  held  either  a  greater  or  a  less  distance  from  the 
eye.  The  size  and  shape  of  the  blind  spot  may  readily  be  determined  as 
follows :  Fix  the  eye  upon  a  definite  point  marked  upon  a  sheet  of  white 
paper.  Bring  the  black  point  of  a  lead  pencil  (which,  except  the  point,  has 
been  painted  white  or  covered  with  white  paper)  into  the  invisible  portion  of 
the  field  of  vision  and  carry  it  outward  in  any  direction  until  it  becomes  vis- 
ible. Mark  upon  the  paper  the  point 
at  which  it  just  begins  to  be  seen,  and 
by  repeating  the  process  in  as  many 
different  directions  as  possible  the  out- 
line of  the  blind  spot  may  be  marked 
out.  Figure  234  shows  the  shape  of 
the  blind  spot  determined  by  Helm- 
holtz  in  his  own  right  eye,  a  being 

FIG.  234.-Form  of  the  blind  spot  (Helmholtz).         the   P°int   °f  fixati°n    °f  the   e7e»  and 

the  line  AB  being  one-third  of  the 
distance  between  the  eye  and  the  paper.     The  irregularities  of  outline,  as  at 


THE  SENSE    OF    VISION. 


775 


d,  are  due  to  shadows  of  the  large  retinal  vessels.  During  this  determination 
it  is  of  course  necessary  that  the  head  should  occupy  a  fixed  position  with 
regard  to  the  paper.  This  condition  can  be  secured  by  holding  firmly  between 
the  teeth  a  piece  of  wood  that  is  clamped  in  a  suitable  position  to  the  edge  of 
the  table.  The  diameter  of  the  blind  spot,  as  thus  determined,  has  been  found 
to  correspond  to  a  visual  angle  varying  from  3°  39'  to  9°  47',  the  average 
measurement  being  6°  10'.  This  is  about  the  angle  that  is  subtended  by  the 
human  face  seen  at  a  distance  of  two  meters.  Although  a  considerable  por- 
tion of  the  retina  is  thus  insensible  to  light,  we  are,  in  the  ordinary  use  of  the 
eyes,  conscious  of  no  corresponding  blank  in  the  field  of  vision.  By  what 
psychical  operation  we  "  fill  up "  the  gap  in  our  subjective  field  of  vision 
caused  by  the  blind  spot  of  the  retina  is  a  question  that  has  been  much  dis- 
cussed without  being  definitely  settled. 

The  above-mentioned  reasons  for  regarding  the  rods  and  cones  as  the  light- 
perceiving  elements  of  the  retina  seem  sufficiently  conclusive.  Whether  there 
is  any  difference  between  the  rods  and  the  cones  with  regard  to  their  light- 
perceiving  function  is  a  question  which  may  be  best  considered  in  connection 
with  a  description  of  the  qualitative  modifications  of  light. 

The  histological  relation  between  the  various  layers  of  the  retina  is  still 
under  discussion.  According  to  recent  observations  of  Cajal,1  the  connection 
between  the  rods  and  cones  on  the  one 
side  and  the  fibres  of  the  optic  nerve 
on  the  other  is  established  in  a  man- 
ner which  is  represented  diagram- 
matically  in  Figure  235.  The  pro- 
longations of  the  bipolar  cells  of  the 
internal  nuclear  layer  E  break  up  into 
fine  fibres  in  the  external  molecular 
(or  plexiform)  layer  C.  Here  they  are 
brought  into  contact,  though  not  into 
anatomical  continuity,  with  the  termi- 
nal fibres  of  the  rods  and  cones.  The 
inner  prolongations  of  the  same  bipolar 
cells  penetrate  into  the  internal  molec- 
ular (or  plexiform)  layer  F,  and  there 
come  into  contact  with  the  dendrites 
coming  from  the  layer  of  ganglion-cells 
G.  These  cells  are,  in  their  turn,  con- 
nected by  their  axis-cylinder  processes 


Rods. 


Cones. 


FIG.  235.— Diagrammatic  representation  of  the 


..,        !        /»i  />    ,1  ..  rrii  r Hi.  &».— uiagrammauc  represeuiauou  ui  MIC 

with  the  fibres  of  the  optic  nerve.    The    structure  of  the  retina  (Cajai):  A,  layer  of  rods 

bipolar  Cells  which  Serve    as    Connective     and  cones;  B,  external  nuclear  layer ;  C,  external 


molecular  (or  plexiform)  layer;  E,  internal  nu- 


1  •       i  1  1  i  i|  .  •  iiiuit-t/LiJLWi     \\.n      picAm-niii./    Lnyui.  ,     A»J    IIILCI  iic*x    ut 

links    between    the    rods    and    the   OptlC     clear  layer .  Ff  internal  molecular  (or  plexiform) 

nerve-fibres    are    anatomically    distin-    layer'  <*.  lftyer  of  gangiion-ceiis :  H,  layer  of 


nerve-fibres. 


.guishable  (as  indicated  in  the  diagram) 

1  Die  Retina  der  Wirbeltkiere,  Wiesbaden,  1894. 


776  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

from  those  which  perform  the  same  function  for  the  cones.  Whatever  be  the 
precise  mode  of  connection  between  the  rods  and  cones  and  the  fibres  of  the 
optic  nerve,  it  is  evident  that  each  retinal  element  cannot  be  connected  with 
the  nerve-centres  by  a  separate  independent  nerve-channel,  since  the  retina 
contains  many  millions  of  rods  and  cones,  while  the  optic  nerve  has  only 
about  438,000  nerve-fibres,1  though  of  course  such  a  connection  may  exist  in 
the  fovea  centralis,  as  Cajal  has  shown  is  probably  the  case  in  reptiles  and  birds. 
Changes  Produced  in  the  Retina  by  Light. — We  must  now  inquire 
what  changes  can  be  supposed  to  occur  in  the  rods  and  cones  under  the  influ- 
ence of  light  by  means  of  which  they  are  able  to  transform  the  energy  of  the 
ether  vibrations  into  a  stimulus  for  the  fibres  of  the  optic  nerve.  Though  in 
the  present  state  of  our  knowledge  no  satisfactory  answer  can  be  given  to  this 
question,  yet  certain  direct  effects  of  light  upon  the  retina  have  been  observed 
which  are  doubtless  associated  in  some  way  with  the  transformation  in 
question. 

The  retina  of  an  eye  which  has  been  protected  from  light  for  a  considerable 
length  of  time  has  a  purplish-red  color,  which  upon  exposure  to  light  changes 
to  yellow  and  then  fades  away.  This  bleaching  occurs  also  in  monochromatic 
light,  the  most  powerful  rays  being  those  of  the  greenish-yellow  portion  of 
the  spectrum — i.  e.  those  rays  which  are  most  completely  absorbed  by  the  pur- 
plish-red coloring  matter.  A  microscopic  examination  of  the  retina  shows 
that  this  coloring  matter,  which  has  been  termed  visual  purple,  is  entirely  con- 
fined to  the  outer  portion  of  the  retinal  rods  and  does  not  occur  at  all  in  the 
cones.  After  being  bleached  by  light  it  is,  during  life,  restored  through  the 
agency  of  the  pigment  epithelium,  the  cells  of  which,  under  the  influence  of 
light,  send  their  prolongations  inward  to  envelop  the  outer  limbs  of  the  rods 
and  cones  with  pigment.  If  an  eye,  either  excised  or  in  its  natural  position, 
is  protected  from  light  for  a  time,  and  then  placed  in  such  a  position  that  the 
image  of  a  lamp  or  a  window  is  thrown  upon  the  retina  for  a  time  which  may 
vary  with  the  amount  of  light  from  seven  seconds  to  ten  minutes,  it  will  be 
found  that  the  retina,  if  removed  and  examined  under  red  light,  will  show  the 
image  of  the  luminous  object  impressed  upon  it  by  the 
bleaching  of  the  visual  purple. 

If  the  retina  be  treated  with  a  4  per  cent,  solution  of 
alum,  the  restoration  of  the  visual  purple  will  be  pre- 
vented, and  the  so-called  "  optogram "  will  be,  as  pho- 
tographers say,  "  fixed." 2 

Figlll>e  236  shows  the  appearance  of  a  rabbit's  retina 
on  which  the  optogram  of  a  window  has  been  impressed. 
Although  the  chemical  changes  in  the  visual  purple  under  the  influence  of 
light  seem,  at  first  sight,  to  afford  an  explanation  of  the  transformation  of  the 
vibrations  of  the  luminiferous  ether  into  a  stimulation  for  the  optic  nerve,  yet 
the  fact  that  vision  is  most  distinct  in  the  fovea  centralis  of  the  retina,  which,, 

1  Salzer:  Wiener  Sitzungsberichte,  1880,  Bd.  Ixxxi.  S.  3. 

2  Kiihne:  Unlersuchungen  a.  d.  phys.  InsL  d.  Universitdt  Heidelberg,  i.  1. 


THE  SENSE    OF    VISION.  777 

as  it  contains  no  rods,  is  destitute  of  visual  purple,  makes  it  impossible  to 
regard  this  coloring  matter  as  essential  to  vision.  The  most  probable  theory 
of  its  function  is  perhaps  that  which  connects  it  with  the  adaptation  of  the 
eye  to  varying  amounts  of  light,  as  described  on  p.  772. 

In  addition  to  the  above-mentioned  movements  of  the  pigment  epithelium 
cells  under  the  influence  of  light,  certain  changes  in  the  retinal  cones  of  frogs 
and  fishes  have  been  observed.1  The  change  consists  in  a  shortening  and  thick- 
ening of  the  inner  portion  of  the  cones  when  illuminated,  but  the  relation  of 
the  phenomenon  to  vision  has  not  been  explained. 

Like  most  of  the  living  tissues  of  the  body,  the  retina  is  the  seat  of  electri- 
cal currents.  In  repose  the  fibres  of  the  optic  nerve  are  said  to  be  positive  in 
relation  to  the  layer  of  rods  and  cones.  When  light  falls  upon  the  retina  this 
current  is  at  first  increased  and  then  diminished  in  intensity. 

Sensation  of  Light. — Whatever  view  may  be  adopted  with  regard  to  the 
mechanism  by  which  light  is  enabled  to  become  a  stimulus  for  the  optic  nerve, 
the  fundamental  fact  remains  that  the  retina  (and  in  all  probability  the  layer 
of  rods  and  cones  in  the  retina)  alone  supplies  the  conditions  under  which  this 
transformation  of  energy  is  possible.  But  in  accordance  with  the  "  law  of 
specific  energy  "  a  sensation  of  light  may  be  produced  in  whatever  way  the 
optic  nerve  be  stimulated,  for  a  stimulus  reaching  the  visual  centres  through 
the  optic  nerve  is  interpreted  as  a  visual  sensation,  in  the  same  way  that 
pressure  on  a  nerve  caused  by  the  contracting  cicatrix  of  an  amputated  leg 
often  causes  a  painful  sensation  which  is  referred  to  the  lost  toes  to  which  the 
nerve  was  formerly  distributed.  Thus  local  pressure  on  the  eyeball  by  stimu- 
lating the  underlying  retina  causes  luminous  sensations,  already  described  as 
"  phosphenes,"  and  electrical  stimulation  of  the  eye  as  a  whole  or  of  the  stump 
of  the  optic  nerve  after  the  removal  of  the  eye  is  found  to  give  rise  to  sensa- 
tions of  light. 

Vibrations  of  the  luminiferous  ether  constitute,  however,  the  normal  stim- 
ulus of  the  retina,  and  we  must  now  endeavor  to  analyze  the  sensation  thus 
produced.  In  the  first  place,  it  must  be  borne  in  mind  that  the  so-called  ether 
waves  differ  among  themselves  very  widely  in  regard  to  their  rate  of  oscilla- 
tion. The  slowest  known  vibrations  of  the  ether  molecules  have  a  frequency 
of  about  107,000,000,000,000  in  a  second,  and  the  fastest  a  rate  of  about 
40,000,000,000,000,000  in  a  second — a  range,  expressed  in  musical  terms,  of 
about  eight  and  one-half  octaves.  All  these  ether  waves  are  capable  of  warm- 
ing bodies  upon  which  they  strike  and  of  breaking  up  certain  chemical  com- 
binations, the  slowly  vibrating  waves  being  especially  adapted  to  produce  the 
former  and  the  rapidly  vibrating  ones  the  latter  effect.  Certain  waves  of 
intermediate  rates  of  oscillation — viz.  those  ranging  between  392,000,000,- 
000,000  and  757,000,000,000,000  in  a  second — not  only  produce  thermic  and 
chemical  effects,  but  have  the  power,  when  they  strike  the  retina,  of  causing 
changes  in  the  layer  of  rods  and  cones,  which,  in  their  turn,  act  as  a  stimulus 
to  the  optic  nerve.  The  ether  waves  which  produce  these  various  phenomena 

1  Engelmann  :  Archivfiir  die  gesammte  Physiologic,  xxxv.  498. 


778  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

are  often  spoken  of  as  heat  rays,  light  rays,  and  actinic  or  chemical  rays,  but 
it  must  be  remembered  that  the  same  wave  may  produce  all  three  classes  of 
phenomena,  the  effect  depending  upon  the  nature  of  the  substance  upon  which 
it  strikes.  It  will  be  observed  that  the  range  of  vibrations  capable  of  affecting 
the  retina  is  rather  less  than  one  octave,  a  limitation  which  obviously  tends  to 
reduce  the  amount  of  chromatic  aberration. 

In  this  connection  it  is  interesting  to  notice  that  the  highest  audible  note  is 
produced  by  about  40,000  sonorous  impulses  in  a  second.  Between  the  high- 
est audible  note  and  the  lowest  visible  color  there  is  a  gap  of  nearly  thirty-four 
octaves  in  which  neither  the  vibrations  of  the  air  nor  those  of  the  luminifer- 
ous  ether  affect  our  senses.  Even  if  the  slowly  vibrating  heat-rays  which 
affect  our  cutaneous  nerves  are  taken  into  account,  there  still  remain  over 
thirty-one  octaves  of  vibrations,  either  of  the  air  or  of  the  luminiferous  ether, 
which  may  be,  and  very  likely  are,  filling  the  universe  around  us  without  in 
any  way  impressing  themselves  upon  our  consciousness. 

Qualitative  Modifications  of  Light. — All  the  ethereal  vibrations  which 
are  capable  of  affecting  the  retina  are  transmitted  with  very  nearly  the  same 
rapidity  through  air,  but  when  they  enter  a  denser  medium  the  waves  having 
a  rapid  vibration  are  retarded  more  than  those  vibrating  more  slowly.  Hence 
when  a  ray  of  sunlight  composed  of  all  the  visible  ether  waves  strikes  upon  a 

plane  surface  of  glass,  the  greater 
retardation  of  the  waves  of  rapid 
vibration  causes  them  to  be  more 
refracted  than  those  of  slower  vibra- 
tion, and  if  the  glass  has  the  form 
of  a  prism,  as  shown  in  Figure  237, 
this  so-called  "  dispersion  "  of  the 
rays  is  still  further  increased  when 
the  rays  leave  the  glass,  so  that  the 
emerging  beam,  if  received  upon  a 

FIG.  237.-Diagram  illustra^mgthe  dispersion  of  light      whjte  surface>  instead  of  forming  a 

spot  of  white  light,  produces  a  band 

of  color  known  as  the  solar  spectrum.  The  colors  of  the  spectrum,  though 
commonly  spoken  of  as  seven  in  number,  really  form  a  continuous  series  from 
the  extreme  red  to  the  extreme  violet,  these  colors  corresponding  to  ether  vibra- 
tions have  rates  of  392,000,000,000,000  and  757,000,000,000,000  in  1  second, 
and  wave  lengths  of  0.7667  and  0.3970  micromillimeters  *  respectively. 

Colors,  therefore,  are  sensations  caused  by  the  impact  upon  the  retina  of 
certain  ether  waves  having  definite  frequencies  and  wave-lengths,  but  these 
are  not  the  only  peculiarities  of  the  ether  vibration  which  influence  the  retinal 
sensation.  The  energy  of  the  vibration,  or  the  vis  viva  of  the  vibrating  mole- 
cule, determines  the  "  intensity  "  of  the  sensation  or  the  brilliancy  of  the  light.2 

^ne  micromillimeter  =  0.001  millimeter  =  one  //. 

2  The  energy  of  vibration  capable  of  producing  a  given  subjective  sensation  of  intensity 
varies  with  the  color  of  the  light,  as  will  be  later  explained  (see  p.  786). 


THE  SENSE    OF    VISION.  779 

Furthermore,  the  sensation  produced  by  the  impact  of  ether  waves  of  a  definite 
length  will  vary  according  as  the  eye  is  simultaneously  affected  by  a  greater  or 
less  amount  of  white  light.  This  modification  of  the  sensation  is  termed  its 
degree  of  "  saturation/7  light  being  said  to  be  completely  saturated  when  it  is 
"  monochromatic"  or  produced  by  ether  vibrations  of  a  single  wave-length. 

The  modifications  of  light  which  taken  together  determine  completely  the 
character  of  the  sensation  are,  then,  three  in  number — viz. :  1.  Color,  depend- 
ent upon  rate  of  vibration  or  length  of  the  ether  wave ;  2.  Intensity,  dependent 
upon  the  energy  of  the  vibration ;  3.  Saturation,  dependent  upon  the  amount 
of  white  light  mingled  with  the  monochromatic  light.  These  three  qualitative 
modifications  of  light  must  now  be  considered  in  detail. 

Color. — In  our  profound  ignorance  of  the  nature  of  the  process  by  which, 
in  the  rods  and  cones,  the  movements  of  the  ether  waves  are  converted  into  a 
stimulus  for  the  optic  nerve-fibres,  all  that  can  be  reasonably  demanded  of  a 
color  theory  is  that  it  shall  present  a  logically  consistent  hypothesis  to  account 
for  the  sensations  actually  produced  by  the  impact  of  ether  waves  of  varying 
rates,  either  singly  or  combined,  upon  different  parts  of  the  retina.  Some  of 
the  important  phenomena  of  color  sensation  of  which  every  color  theory  must 
take  account  may  be  enumerated  as  follows  : 

1.  Luminosity  is  more  readily  recognized  than  color.     This  is  shown  by 
the  fact  that  a  colored  object  appears  colorless  when  it  is  too  feebly  illuminated, 
and  that  a  spectrum  produced  by  a  very  feeble  light  shows  variations  of  inten- 
sity with  a  maximum  nearer  than  normal  to  the  blue  end,  but  no  gradations 
of  color.     A  similar  lack  of  color  is  noticed  when  a  colored  object  is  observed 
for  too  short  a  time  or  when  it  is  of  insufficient  size.     In  all  these  respects  the 
various  colors  present  important  individual  differences  which  will  be  considered 
later, 

2.  Colored  objects  seen  with  increasing  intensity  of  illumination  appear 
more  and  more  colorless,  and  finally  present  the  appearance  of  pure  white. 
Yellow  passes  into  white  more  readily  than  the  other  colors. 

3.  The  power  of  the  retina  to  distinguish  colors  diminishes  from  the  centre 
toward  the  periphery,  the  various  colors,  in  this  respect  also,  differing  mate- 
rially from  each  other.     Sensibility  to  red  is  lost  at  a  short  distance  from  the 
macula  lutea,  while  the  sensation  of  blue  is  lost  only  on  the  extreme  lateral 
portions  of  the  retina.     The  relation  of  this  phenomenon  to  the  distribution 
of  the  rods  and  cones  in  the  retina  will  be  considered  in  connection  with  the 
perception  of  the  intensity  of  light. 

Color-mixture. — Since  the  various  spectral  colors  are  produced  by  the  dis- 
persion of  the  white  light  of  the  sun,  it  is  evident  that  white  light  may  be 
reproduced  by  the  reunion  of  the  rays  corresponding  to  the  different  colors,  and 
it  is  accordingly  found  that  if  the  colored  rays  emerging  from  a  prism,  as  in 
Fig.  237,  are  reunited  by  suitable  refracting  surfaces,  a  spot  of  white  light  will  be 
produced  similar  to  that  which  would  have  been  caused  by  the  original  beam 
of  sunlight.  But  white  light  may  be  produced  not  only  by  the  union  of  all 
the  spectral  colors,  but  by  the  union  of  certain  selected  colors  in  twos,  threes, 


780  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

fours,  etc.  Any  two  spectral  colors  which  by  their  union  produce  white  are 
said  to  be  "  complementary  "  colors.  The  relation  of  these  pairs  of  comple- 
mentary colors  to  each  other  may  be  best  understood  by  reference  to  Figure  238. 


p 

FIG.  238.— Color  diagram. 

Here  the  spectral  colors  are  supposed  to  be  disposed  around  a  curved  line, 
as  indicated  by  their  initial  letters,  and  the  two  ends  of  the  curve  are  united 
by  a  straight  line,  thus  enclosing  a  surface  having  somewhat  the  form  of  a  tri- 
angle with  a  rounded  apex.  If  the  curved  edge  of  this  surface  be  supposed  to 
be  loaded  with  weights  proportionate  to  the  luminosity  of  the  different  colors, 
the  centre  of  gravity  of  the  surface  will  be  near  the  point  W.  Now,  if  a 
straight  line  be  drawn  from  any  point  on  the  curved  line  through  the  point 
JFand  prolonged  till  it  cuts  the  curve  again,  the  colors  corresponding  to  the 
two  ends  of  this  straight  line  will  be  complementary  colors.  Thus  in  Figure 
238  it  will  be  seen  that  the  complementary  color  of  red  is  bluish-green,  and 
that  of  yellow  lies  near  the  indigo.  It  is  also  evident  that  the  complementary 
color  of  green  is  purple,  which  is  not  a  spectral  color  at  all,  but  a  color 
obtained  by  the  union  of  violet  and  red.  The  union  of  a  pair  of  colors 
lying  nearer  together  than  complementary  colors  produces  an  intermediate  color 
mixed  with  an  amount  of  white  which  is  proportionate  to  the  nearness  of  the 
colors  to  the  complementary.  Thus  the  union  of  red  and  yellow  produces 
orange,  but  a  less  saturated  orange  than  the  spectral  color.  The  union  of  two 
colors  lying  farther  apart  than  complementary  colors  produces  a  color  which 
borders  more  or  less  upon  purple. 

The  mixing  of  colors  to  demonstrate  the  above-mentioned  effects  may  be 
accomplished  in  three  different  ways  : 

1.  By  employing  two  prisms  to  produce  two  independent  spectra,  and  then 
directing  the  colored  rays  which  are  to  be  united  so  that  they  will  illuminate 
the  same  white  surface. 

2.  By  looking  obliquely  through  a  glass  plate  at  a  colored  object  placed 
behind  it,  while  at  the  same  time  light  from  another  colored  object,  placed  in 
front  of  the  glass,  is  reflected  into  the  eye  of  the  observer,  as  shown  in  Figure 
239.     Here  the  transmitted  light  from  the  colored  object  A  and  the  reflected 
light  from  the  colored  object  B  enter  the  eye  at  C  from  the  same  direction, 
and  are  therefore  united  upon  the  retina. 

3.  By  rotating  before  the  eye  a  disk  on  which  the  colors  to  be  united  are 


THE  SENSE    OF    VISION.  781 

painted  upon  different  sectors.  This  is  most  readily  accomplished  by  using 
a  number  of  disks,  each  painted  with  one  of  the  colors  to  be  experimented 
with,  and  each  divided  radially  by  a  cut  running  from  the  centre  to  the  circum- 
ference. The  disks  can  then  be  lapped  over  each  other  and  rotated  together,  and 
in  this  way  two  or  more  colors  can  be  mixed  in  any  desired  proportions.  This 
method  of  mixing  colors  depends  upon 
the  property  of  the  retina  to  retain  an 
impression  after  the  stimulus  causing  [V 


it  has  ceased  to  act — a  phenomenon  of  /  "  \ 

great  importance  in  physiological  optics,  / 

and  one  which  will  be  further  discussed  / 


in  connection  with  the  subject  of  "  after-  /  \ 


images." 


_ 

The    physiological    mixing  of  Colors      FIG.  239.—  Diagram  to  illustrate  color  mixture  by 

cannot  be  accomplished  by  the  mixture 

of  pigments  or  by  allowing  sunlight  to  pass  successively  through  glasses  of 
different  colors,  for  in  these  cases  rays  corresponding  to  certain  colors  are 
absorbed  by  the  medium  through  which  the  white  light  passes,  and  the  phe- 
nomenon is  the  result  of  a  process  of  subtraction  and  not  addition.  Light 
reaching  the  eye  through  red  glass,  for  instance,  looks  red  because  all  the  rays 
except  the  red  rays  are  absorbed,  and  light  coming  through  green  glass  appears 
green  for  a  similar  reason.  Now,  when  light  is  allowed  to  pass  successively 
through  red  and  green  glass  the  only  rays  which  pass  through  the  red  glass 
will  be  absorbed  by  the  green.  Hence  no  light  will  pass  through  the  combi- 
nation of  red  and  green  glass,  and  darkness  results.  But  when  red  and  green 
rays  are  mixed  by  any  of  the  three  methods  above  described  the  result  of  this 
process  of  addition  is  not  darkness,  but  a  yellow  color,  as  will  be  understood 
by  reference  to  the  color  diagram  on  p.  780.  In  the  case  of  colored  pigments 
similar  phenomena  occur,  for  here  too  light  reaches  the  eye  after  rays  of  cer- 
tain wave-lengths  have  been  absorbed  by  the  medium.  This  subject  will  be 
further  considered  in  connection  with  color-theories. 

Color-theories.  —  From  what  has  been  said  of  color-mixtures  it  is  evident 
that  every  color  sensation  may  be  produced  by  the  mixture  of  a  number  of 
other  color  sensations,  and  that  certain  color  sensations  —  viz.  the  purples  —  can 
be  produced  only  by  the  mixture  of  other  sensations,  since  there  is  no  single 
wave-length  corresponding  to  them.  Hence  the  hypothesis  is  a  natural  one 
that  all  colors  are  produced  by  the  mixture  in  varying  proportions  of  a  certain 
number  of  fundamental  colors,  each  of  which  depends  for  its  production  upon 
the  presence  in  the  retina  of  a  certain  substance  capable  of  being  affected 
(probably  through  some  sort  of  a  photo-chemical  process)  by  light  of  a  certain 
definite  wave-length.  A  hypothesis  of  this  sort  lies  at  the  basis  of  both  the 
Young-Helmholtz  and  the  Hering  theories  of  color  sensation. 

The  former  theory  postulates  the  existence  in  the  retina  of  three  substances 
capable  of  being  affected  by  red,  green,  and  violet  rays,  respectively  —  i.  e.  by 
the  three  colors  lying  at  the  three  angles  of  the  color  diagram  given  on  p.  780 


782  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

— and  regards  all  other  color  sensations  as  produced  by  the  simultaneous  affec- 
tion of  two  of  these  substances  in  varying  proportions.  Thus  when  a  ray  of 
blue  light  falls  on  the  retina  it  stimulates  the  violet-  and  green-perceiving  sub- 
stances, and  produces  a  sensation  intermediate  between  the  two,  while  simul- 
taneous stimulation  of  the  red-  and  green-perceiving  substances  produces  the 
sensations  corresponding  to  yellow  and  orange ;  and  when  the  violet-  and  red- 
perceiving  substances  are  affected  at  the  same  time,  the  various  shades  of 
purple  are  produced.  Each  of  these  three  substances  is,  however,  supposed  to 
be  affected  to  a  slight  extent  by  all  the  rays  of  the  visible  spectrum,  a  suppo- 
sition which  is  rendered  necessary  by  the  fact  that  even  the  pure  spectral 
colors  do  not  appear  to  be  perfectly  saturated,  as  will  be  explained  in  connec- 
tion with  the  subject  of  saturation.  Furthermore,  the  disappearance  of  color 
when  objects  are  very  feebly  or  very  brightly  illuminated  or  when  they  are 
seen  with  the  lateral  portions  of  the  retina  (as  described  on  p.  779)  necessitates 
the  additional  hypotheses  that  these  three  substances  are  all  equally  affected  by 
all  kinds  of  rays  when  the  light  is  of  either  very  small  or  very  great  intensity 
or  when  it  falls  on  the  extreme  lateral  portions  of  the  retina,  and  that  they 
manifest  their  specific  irritability  for  red,  green,  and  violet  rays  respectively 
only  in  light  of  moderate  intensity  falling  not  too  far  from  the  fovea  centralis 
of  the  retina. 

The  modifications  of  the  Young- Hemholtz  theory  introduced  by  these  sub- 
sidiary hypotheses  greatly  diminish  the  simplicity  which  was  its  chief  claim  to 
acceptance  when  originally  proposed.  Moreover,  there  will  always  remain  a 
psychological  difficulty  in  supposing  that  three  sensations  so  different  from  each 
other  as  those  of  red,  green,  and  violet  can  by  their  union  produce  a  fourth 
sensation  absolutely  distinct  from  any  of  them — viz.  white. 

The  fact  that  in  the  Hering  theory  this  difficulty  is  obviated  has  contributed 
greatly  to  its  acceptance  by  physiologists.  In  this  theory  the  retina  is  supposed 
to  contain  three  substances  in  which  chemical  changes  may  be  produced  by  ether 
vibrations,  but  each  of  these  substances  is  supposed  to  be  affected  in  two  oppo- 
site ways  by  rays  of  light  which  correspond  to  complementary  color  sensa- 
tions. Thus  in  one  substance — viz.  the  white-black  visual  substance — kata- 
bolic  or  destructive  changes  are  supposed  to  be  produced  by  all  the  rays  of  the 
visible  spectrum,  the  maximum  effect  being  caused  by  the  yellow  rays,  while 
anabolic  or  constructive  changes  occur  when  no  light  at  all  falls  upon  the 
retina.  The  chemical  changes  of  this  substance  correspond,  therefore,  to  the 
sensation  of  luminosity  as  distinguished  from  color.  In  a  second  substance  red 
rays  are  supposed  to  produce  katabolic,  and  green  rays  anabolic  changes,  while 
a  third  substance  is  similarly  affected  by  yellow  and  blue  rays.  These  two 
substances  are  therefore  spoken  of  as  red-green  and  yellow-blue  visual  sub- 
stances respectively. 

It  has  been  sometimes  urged  as  an  objection  to  this  theory  that  the  effect  of 
a  stimulus  is  usually  katabolic  and  not  anabolic.  This  is  true  with  regard  to 
muscular  contraction,  from  the  study  of  which  phenomenon  most  of  our  know- 
ledge of  the  effect  of  stimulation  has  been  obtained,  but  it  should  be  remem- 


THE  SENSE    OF    VISION.  783 

bered  that  observations  on  the  augmentor  and  inhibitory  cardiac  nerves  have 
shown  us  that  nerve-stimulation  may  produce  very  contrary  effects.  There 
seems  to  be,  therefore,  no  serious  theoretical  difficulty  in  supposing  that  light 
rays  of  different  wave-lengths  may  produce  opposite  metabolic  effects  upon  the 
substances  in  which  changes  are  associated  with  visual  sensations. 

A  more  serious  objection  lies  in  the  difficulty  of  distinguishing  between  the 
sensation  of  blackness,  which,  on  Bering's  hypothesis,  must  correspond  to  active 
anabolism  of  the  white-black  substance,  and  the  sensation  of  darkness  (such  as 
we  experience  when  the  eyes  have  been  withdrawn  for  some  time  from  the 
influence  of  light),  which  must  correspond  to  a  condition  of  equilibrium  of 
the  white-black  substance  in  which  neither  anabolism  nor  katabolism  is 
occurring. 

Another  objection  to  the  Hering  theory  is  to  be  found  in  the  results  of 
experiments  in  comparing  grays  or  whites  produced  by  mixing  different  colored 
rays  under  varying  intensities  of  light.  The  explanation  given  by  Hering  of 
the  production  of  white  through  the  mixture  of  blue  and  yellow  or  of  red  and 
green  is  that  when  either  of  these  pairs  of  complementary  colors  is  mixed 
the  anabolic  and  the  katabolic  processes  balance  each  other,  leaving  the  corre- 
sponding visual  substance  in  a  condition  of  equilibrium.  Hence,  the  white- 
black  substance  being  alone  stimulated,  the  result  will  be  a  sensation  of  white 
corresponding  to  the  intensity  of  the  katabolic  process  caused  by  the  mixed 
rays.  Now,  it  is  found  that  when  blue  and  yellow  are  mixed  in  certain  pro- 
portions on  a  revolving  disk  a  white  can  be  produced  which  will,  with  a  certain 
intensity  of  illumination,  be  undistinguishable  from  a  white  produced  by  mix- 
ing red  and  green.  If,  however,  the  intensity  of  the  illumination  is  changed, 
it  will  be  found  necessary  to  add  a  certain  amount  of  white  to  one  of  the  mix- 
tures in  order  to  bring  them  to  equality.  On  the  theory  that  complementary 
colors  produce  antagonistic  processes  in  the  retina  it  is  difficult  to  understand 
why  this  should  be  the  case. 

A  color  theory  which  is  in  some  respects  more  in  harmony  with  recent 
observations  in  the  physiology  of  vision  has  been  proposed  by  Mrs.  C.  L. 
Franklin.  In  this  theory  it  is  supposed  that,  in  its  earlier  periods  of  de- 
velopment, the  eye  is  sensitive  only  to  luminosity  and  not  to  color — i.  e.  it 
possesses  only  a  white-black  or  (to  use  a  single  word)  a  gray-perceiving  sub- 
stance which  is  affected  by  all  visible  light  rays,  but  most  powerfully  by  those 
lying  near  the  middle  of  the  spectrum.  The  sensation  of  gray  is  supposed  to 
be  dependent  upon  the  chemical  stimulation  of  the  optic  nerve-terminations  by 
some  product  of  decomposition  of  this  substance. 

In  the  course  of  development  a  portion  of  this  gray  visual  substance  becomes 
differentiated  into  three  different  substances,  each  of  which  is  affected  by  rays 
of  light  corresponding  to  one  of  the  three  fundamental  colors  of  the  spectrum 
— viz.  red,  green,  and  blue.  When  a  ray  of  light  intermediate  between  two 
of  the  fundamental  colors  falls  upon  the  retina,  the  visual  substances  corre- 
sponding to  these  two  colors  will  be  affected  to  a  degree  proportionate  to  the 
proximity  of  these  two  colors  to  that  of  the  incident  ray.  Since  this  effect  is 


784  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

exactly  the  same  as  that  which  is  produced  when  the  retina  is  acted  upon  simul- 
taneously by  light  of  two  fundamental  colors,  we  are  incapable  of  distinguish- 
ing in  sensation  between  an  intermediate  wave-length  and  a  mixture  in  proper 
amounts  of  two  fundamental  wave-lengths. 

When  the  retina  is  affected  by  two  or  more  rays  of  such  wave-lengths  that 
all  three  of  the  color  visual  substances  are  equally  affected,  the  resulting  decom- 
position will  be  the  same  as  that  produced  by  the  stimulation  of  the  gray  visual 
substance  out  of  which  the  color  visual  substances  were  differentiated,  and  the 
corresponding  sensation  will  therefore  be  that  of  gray  or  white. 

It  will  be  noticed  that  the  important  feature  of  this  theory  is  that  it  pro- 
vides for  the  independent  existence  of  the  gray  visual  substance,  while  at  the 
same  time  the  stimulation  of  this  substance  is  made  a  necessary  result  of  the 
mixture  of  certain  color  sensations. 

Color-blindness. — The  fact  that  many  individuals  are  incapable  of  distin- 
guishing between  certain  colors — i.  e.  are  more  or  less  "  color-blind  " — is  one 
of  fundamental  importance  in  the  discussion  of  theories  of  color  vision.  By 
far  the  most  common  kind  of  color-blindness  is  that  in  which  certain  shades 
of  red  and  green  are  not  recognized  as  different  colors.  The  advocates  of  the 
Young-Helmholtz  theory  explain  such  cases  by  supposing  that  either  the  red 
or  the  green  perceiving  elements  of  the  retina  are  deficient,  or,  if  present,  are 
irritable,  not  by  rays  of  a  particular  wave-length,  but  by  all  the  rays  of  the 
visible  spectrum.  In  accordance  with  this  view  these  cases  of  color-blindness 
are  divided  into  two  classes — viz.  the  red-blind  and  the  green-blind— the  basis 
for  the  classification  being  furnished  by  more  or  less  characteristic  curves  repre- 
senting the  variations  in  the  luminosity  of  the  visible  spectrum  as  it  appears 
to  the  different  eyes.  There  are,  however,  cases  which  cannot  easily  be  brought 
under  either  of  these  two  classes.  Moreover,  it  has  been  proved  in  cases  of 
monocular  color-blindness,  and  is  admitted  even  by  the  defenders  of  the  Helm- 
holtz  theory,  that  such  persons  see  really  only  two  colors — viz.  blue  and  yellow. 
To  such  persons  the  red  end  of  the  spectrum  appears  a  dark  yellow,  and  the 
green  portion  of  the  spectrum  has  luminosity  without  color. 

A  better  explanation  of  this  sort  of  color-blindness  is  given  in  the  Hering 
theory  by  simply  supposing  that  in  such  eyes  the  red-green  visual  substance  is 
deficient  or  wholly  wanting,  but  the  theory  of  Mrs.  Franklin  accounts  for  the 
phenomena  in  a  still  more  satisfactory  way ;  for,  by  supposing  that  the  differ- 
entiation of  the  primary  gray  visual  substance  has  first  led  to  the  formation 
of  a  blue  and  a  yellow  visual  substance,  and  that  the  latter  has  subsequently 
been  differentiated  into  a  red  and  a  green  visual  substance,  color-blindness  is 
readily  explained  by  supposing  that  this  second  differentiation  has  either  not 
occurred  at  all  or  has  taken  place  in  an  imperfect  manner.  It  is,  in  other 
words,  an  arrest  of  development. 

Cases  of  absolute  color-blindness  are  said  to  occasionally  occur.  To  such 
persons  nature  is  colorless,  all  objects  presenting  simply  differences  of  light 
and  shade. 

In  whatever  way  color-blindness  is  to  be  explained,  the  defect  is  one  of 


THE  SENSE    OF    VISION.  785 

considerable  practical  importance,  since  it  renders  those  affected  by  it  incapable 
of  distinguishing  the  red  and  green  lights  ordinarily  used  for  signals.  Such 
persons  are,  therefore,  unsuitable  for  employment  as  pilots,  railway  engineers, 
etc.,  and  it  is  now  customary  to  test  the  vision  of  all  candidates  for  employment 
in  such  situations.  It  has  been  found  that  no  satisfactory  results  can  be 
reached  by  requiring  persons  to  name  colors  which  are  shown  them,  and  the 
chromatic  sense  is  now  commonly  tested  by  what  is  known  as  the  "  Holmgren 
method,"  which  consists  in  requiring  the  individual  examined  to  select  from  a 
pile  of  worsteds  of  various  colors  those  shades  which  seem  to  him  to  resemble 
standard  skeins  of  green  and  pink.  When  examined  in  this  way  about  4  per  \v  1 
cent,  of  the  male  and  one-quarter  of  1  per  cent,  of  the  female  sex  are  found  to^ 
be  more  or  less  color-blind.  The  defect  may  be  inherited,  and  the  relatives 
of  a  color-blind  person  are  therefore  to  be  tested  with  special  care.  Since 
females  are  less  liable  to  be  affected  than  males,  it  often  happens  that  the 
daughters  of  a  color-blind  person,  themselves  with  normal  vision,  have  sons 
who  inherit  their  grandfather's  infirmity. 

Although  in  all  theories  of  color  vision  the  different  sensations  are  supposed 
to  depend  upon  changes  produced  by  the  ether  vibrations  of  varying  rates 
acting  upon  different  substances  in  the  retina,  yet  it  should  be  borne  in  mind 
that  we  have  at  present  no  proof  of  the  existence  of  any  such  substances.  The 
visual  purple — or,  to  adopt  Mrs.  Franklin's  more  appropriate  term,  "  the  rod 
pigment" — was  at  one  time  thought  to  be  such  a  substance,  but  for  the  reasons 
above  given  cannot  be  regarded  as  essential  to  vision.1 

That  a  centre  for  color  vision,  distinct  from  the  visual  centre,  exists  in  the 
cerebral  cortex  is  rendered  probable  by  the  occurrence  of  cases  of  hemianopsia 
for  colors,  and  also  by  the  experiments  of  Heidenhain  and  Cohn  on  the  influ- 
ence of  the  hypnotic  trance  upon  color-blindness. 

Intensity. — The  second  of  the  above-mentioned  qualitative  modifications  of 
light  is  its  intensity,  which  is  dependent  upon  the  energy  of  vibrations  of  the 
molecules  of  the  luminiferous  ether.  The  sensation  of  luminosity  is  not,  how- 
ever, proportionate  to  the  intensity  of  the  stimulus,  but  varies  in  such  a  way 
that  a  given  increment  of  intensity  causes  a  greater  difference  in  sensation  with 
feeble  than  with  strong  illuminations.  This  phenomenon  is  illustrated  by  the 
disappearance  of  a  shadow  thrown  by  a  candle  in  a  darkened  room  on  a  sheet 
of  white  paper  when  sunlight  is  allowed  to  fall  on  the  paper  from  the  opposite 
direction.  In  this  case  the  absolute  difference  in  luminosity  between  the 
shadowed  and  unshadowed  portions  of  the  paper  remains  the  same,  but  it 
becomes  imperceptible  in  consequence  of  the  increased  total  illumination. 

Although  our  power  of  distinguishing  absolute  differences  in  luminosity 
diminishes  as  the  intensity  of  the  illumination  increases,  yet  with  regard  to 
relative  differences  no  such  dependence  exists.  On  the  contrary,  it  is  found 
within  pretty  wide  limits  that,  whatever  be  the  intensity  of  the  illumination, 

1  In  a  recently  developed  theory  by  Ebbinghaus  (Zeitschrtft  fur  Psychologie  und  Physiologic 
der  Sinnesorgane,  v.  145)  a  physiological  importance  in  relation  to  vision  is  attached  to  this 
substance  in  connection  with  other  substances  of  a  hypothetical  character. 
50 


786 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


it  must  be  increased  by  a  certain  constant  fraction  of  its  total  amount  in  order 
to  produce  a  perceptible  difference  in  sensation.  This  is  only  a  special  case  of 
a  general  law  of  sensation  known  as  Weber's  law,  which  has  been  formulated 
by  Foster  as  follows  :  "  The  smallest  change  in  the  magnitude  of  a  stimulus 
which  we  can  appreciate  through  a  change  in  our  sensation  always  bears  the 
same  proportion  to  the  whole  magnitude  of  the  stimulus." 

Luminosity  of  Different  Colors. — When  two  sources  of  light  having  the 
same  color  are  compared,  it  is  possible  to  estimate  their  relative  luminosity 
with  considerable  accuracy,  a  difference  of  about  1  per  cent,  of  the  total 
luminosity  being  appreciated  by  the  eye.  When  the  sources  of  light  have 
different  colors,  much  less  accuracy  is  attainable,  but  there  is  still  a  great  differ- 
ence in  the  intensity  with  which  rays  of  light  of  different  wave-lengths  affect 
the  retina.  We  do  not  hesitate  to  say,  for  instance,  that  the  maximum 
intensity  of  the  solar  spectrum  is  found  in  the  yellow  portion,  but  it  is  import- 
ant to  observe  that  the  position  of  this  maximum  varies  with  the  illumina- 
tion. In  a  very  brilliant  spectrum  the  maximum  shifts  toward  the  orange, 
and  in  a  feeble  spectrum  (such  as  may  be  obtained  by  narrowing  the  slit  of 
the  spectroscope)  it  moves  toward  the  green.  The  curves  in  Figure  240  illus- 


:;.*- 
:',6- 
:$.4- 

Li- 
ft, 

2.H. 

2.6 

2.4 

2.2 

1. 

1.8 

!.«•> 

1.4 

1.2 

1 

0.8 

0.6 

0.4 

0.2 


Intensity  H 
G 
F 
E 
D 
C 
B 
A 


450 


430 
O 


670  660  625  605  590  575     555     535      520     505 
BCD  E  F 

FIG.  240.— Diagram  showing  the  distribution  of  the  intensity  of  the  spectrum  as  dependent  upon  the 

degree  of  illumination  (Konig). 

trate  this  shifting  of  the  maximum  of  luminosity  of  the  spectrum  with  vary- 
ing intensities  of  illumination.  The  abscissas  represent  wave-lengths  in 
millionths  of  a  millimeter,  and  the  ordinates  the  luminosity  of  the  different 
colors  as  expressed  by  the  reciprocal  values  of  the  width  of  the  slit  necessary 
to  give  to  the  color  under  observation  a  luminosity  equal  to  that  of  an  arbi- 


THE  SENSE    OF    VISION.  787 

trarily  chosen  standard.  The  curves  from  A  to  H  represent  the  distribution 
of  the  intensity  of  light  in  the  spectrum  with  eight  different  grades  of  illumi- 
nation. This  shifting  of  the  maximum  of  luminosity  in  the  spectrum 
explains  the  so-called  "  Purkinje's  phenomenon " — viz.  the  changing  rela- 
tive values  of  colors  in  varying  illumination.  This  can  be  best  observed 
at  nightfall,  the  attention  being  directed  to  a  carpet  or  a  wall-paper 
the  pattern  of  which  is  made  up  of  a  number  of  different  colors.  As 
the  daylight  fades  away  the  red  colors,  which  in  full  illumination  are 
the  most  intense,  become  gradually  darker,  and  are  scarcely  to  be  distin- 
guished from  black  at  a  time  when  the  blue  colors  are  still  very  readily 
distinguished. 

Function  of  Rods  and  Cones. — The  layer  of  rods  and  cones  has  thus  far 
been  spoken  of  as  if  all  its  elements  had  one  and  the  same  function.  There 
is,  however,  some  reason  to  suppose  that  the  rods  and  cones  have  different 
functions.  That  color  sensation  and  accuracy  of  definition  are  most  perfect 
in  the  central  portion  of  the  retina  is  shown  by  the  fact  that  when  we  desire 
to  obtain  the  best  possible  idea  of  the  form  and  color  of  an  object  we  direct 
our  eyes  in  such  a  way  that  the  image  falls  upon  the  fovea  centralis  of  the 
retina.  The  luminosity  of  a  faint  object,  however,  seems  greatest  when  we 
look  not  directly  at  it,  but  a  little  to  one  side  of  it.  This  can  be  readily 
observed  when  we  look  at  a  group  of  stars,  as,  for  example,  the  Pleiades. 
When  the  eyes  are  accurately  directed  to  the  stars  so  as  to  enable  us  to  count 
them,  the  total  luminosity  of  the  constellation  appears  much  less  than  when 
the  eyes  are  directed  to  a  point  a  few  degrees  to  one  side  of  the  object.  Now, 
an  examination  of  the  retina  shows  only  cones  in  the  fovea  centralis.  In  the 
immediately  adjacent  parts  a  small  number  of  rods  are  found  mingled  with 
the  cones.  In  the  lateral  portions  of  the  retina  the  rods  are  relatively  more 
numerous  than  the  cones,  and  in  the  extreme  peripheral  portions  the  rods  alone 
exist.  Hence  this  phenomenon  is  readily  explained  on  the  supposition  that 
the  rods  are  a  comparatively  rudimentary  form  of  visual  apparatus  taking 
cognizance  of  the  existence  of  light  with  special  reference  to  its  varying 
intensity,  and  that  the  cones  are  organs  specially  modified  for  the  localization 
of  stimuli  and  for  the  perception  of  differences  of  wave-lengths.  The  view 
that  the  rods  are  specially  adapted  for  the  perception  of  luminosity  and  the 
<x>nes  for  that  of  color  derives  support  from  the  fact  that  in  the  retina  of  cer- 
tain nocturnal  animals — e.  g.  bats  and  owls — rods  alone  are  present.  This 
theory  has  been  further  developed  by  Von  Kries,1  who  in  a  recent  article 
describes  the  rods  as  differing  from  the  cones  in  the  following  respects :  (1) 
They  are  color-blind — i.  e.  they  produce  a  sensation  of  simple  luminosity 
whatever  be  the  wave-length  of  the  light-ray  falling  on  them  ;  (2)  they  are 
more  easily  stimulated  than  the  cones,  and  are  particularly  responsive  to  light- 
waves of  short  wave-lengths ;  (3)  they  have  the  power  of  adapting  themselves  \ 
to  light  of  varying  intensity. 

On  this  theory  it  is  evident  that  we  must  get  the  sensation  of  white  or 

1  Zeiischrift  fiir  Psychologic  und  Physiologic  der  Sinneswgane,  ix.  81. 


788  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

colorless  light  in  two  different  ways  :  (1)  In  consequence  of  the  stimulation 
of  the  rods  by  any  sort  of  light-rays,  and  (2)  in  consequence  of  the  stimula- 
tion of  the  cones  by  certain  combinations  of  light-rays — i.  e.  complementary 
colors.  In  this  double  mode  of  white  perception  lies  perhaps  the  explanation 
of  the  effect  of  varying  intensity  of  illumination  upon  the  results  of  color- 
mixtures  which  has  been  above  alluded  to  (see  p.  783)  as  an  objection  to  the 
Hering  theory.  The  so-called  "  Purkinje's  phenomenon,"  described  on  p.  787, 
is  readily  explained  in  accordance  with  this  theory,  for,  owing  to  the  greater 
irritability  of  the  rods,  the  importance  of  these  organs,  as  compared  with  the 
cones,  in  the  production  of  the  total  visual  sensation  is  greater  with  feeble 
than  with  strong  illumination  of  the  field  of  vision.  At  the  same  time,  the 
power  of  the  rods  to  respond  particularly  to  light-rays  of  short  wave-length 
will  cause  a  greater  apparent  intensity  of  the  colors  at  the  blue  than  at  the  red 
end  of  the  spectrum.  In  this  connection  it  is  interesting  to  note  that  the  phe- 
nomenon is  said  not  to  occur  when  the  observation  is  limited  to  the  fovea 
centralis,  where  cones  alone  are  found.1 

Saturation. — The  degree  of  saturation  of  light  of  a  given  color  depends,  as 
above  stated,  upon  the  amount  of  white  light  mixed  with  it.  The  quality  of 
light  thus  designated  is  best  studied  and  appreciated  by  means  of  experiments 
with  rotating  disks.  If,  for  instance,  a  disk  consisting  of  a  large  white  and  a 
small  red  sector  be  rapidly  rotated,  the  effect  produced  is  that  of  a  pale  pink 
color.  By  gradually  increasing  the  relative  size  of  the  red  sector  the  pink 
color  becomes  more  and  more  saturated,  and  finally  when  the  white  sector  is 
reduced  to  zero  the  maximum  of  saturation  is  produced.  It  must  be  borne 
in  mind,  however,  that  no  pigments  represent  completely  saturated  colors. 
Even  the  colors  of  the  spectrum  do  not  produce  a  sensation  of  absolute 
saturation,  for,  whatever  theory  of  color  vision  be  adopted,  it  is  evident  that 
all  the  color-perceiving  elements  of  the  retina  are  affected  more  or  less  by  all 
the  rays  of  light.  Thus  when  rays  of  red  light  fall  upon  the  retina  they  will 
stimulate  not  only  the  red-perceiving  elements,  but  to  a  slight  extent  also  (to 
use  the  language  of  the  Helmholtz  theory)  the  green-  and  violet-perceiving 
elements  of  the  retina.  The  effect  of  this  will  be  that  of  mixing  a  small 
amount  of  white  with  a  large  amount  of  red  light — i.  e.  it  will  produce  the 
sensation  of  incompletely  saturated  red  light.  This  dilution  of  the  sensation 
can  be  avoided  only  by  previously  exhausting  the  blue-  and  green-perceiving 
elements  of  the  retina  in  a  manner  which  will  be  explained  in  connection  with 
the  phenomena  of  after-images. 

Retinal  Stimulation. — Whenever  by  a  stimulus  applied  to  an  irritable 
substance  the  potential  energy  there  stored  up  is  liberated  the  following  phe- 
nomena may  be  observed  :  1.  A  so-called  latent  period  of  variable  duration 
during  which  no  effects  of  stimulation  are  manifest ;  2.  A  very  brief  period 
during  which  the  effect  of  the  stimulation  reaches  a  maximum ;  3.  A  period 
of  continued  stimulation  during  which  the  effect  diminishes  in  consequence  of 
the  using  up  of  the  substance  containing  the  potential  energy — i.  e.  a  period 

1  Von  Kries  :   Centralblatt  fur  Physiologic,  1896,  i. 


THE  SENSE    OF    VISION. 


789 


of  fatigue ;  4.  A  period  after  the  stimulation  has  ceased  in  which  the  effect 
slowly  passes  away. 


FIG.  241.— Diagram  showing  the  effect  of  stimulation  of  an  irritable  substance. 

The  curve  drawn  by  a  muscle  in  tetanic  contraction,  as  shown  in  Figure 

241,  illustrates  this  phenomenon.    Thus,  if  A  D  represents  the  duration  of  the 
stimulation,  A  B  indicates  the  latent  period,  B  C  the  period  of  contraction, 
C  D  the  period   of  fatigue  under  stimulation,  and  D  E  the  after-effect  of 
stimulation  showing  itself  as  a  slow  relaxation.     When  light  falls  upon  the 
retina  corresponding  phenomena  are  to  be  observed. 

Latent  Period. — That  there  is  a  period  of  latent  sensation  in  the  retina 
(i.  e.  an  interval  between  the  falling  of  light  on  the  retina  and  the  beginning 
of  the  sensation)  is,  judging  from  the  analogy  of  other  parts  of  the  nervous 
system,  quite  probable,  though  its  existence  has  not  been  demonstrated. 

Rise  to  Maximum  of  Sensation. — The  rapidity  with  which  the  sensation  of 
light  reaches  its  maximum  increases  with  the  intensity  of  the  light  and  varies 
with  its  color,  red  light  producing  its  maximum  sensation  sooner  than  green 
and  blue.  Consequently,  when  the  image  of  a  white  object  is  moved  across 
the  retina  it  will  appear  bordered  by  colored  fringes,  since  the  various  con- 
stituents of  white  light  do  not  produce  their  maximum  effects  at  the  same 
time.  This  phenomena  can  be  readily  observed  when  a  disk  on  which  a 
black  and  a  white  spiral  band  alternate  with  each  other  (as  shown  in  Figure 

242,  A)  is  rotated  before  the  eyes.     The  white  band  as  its  image  moves  out- 


FIG.  242.— Disks  to  illustrate  the  varying  rate  at  which  colors  rise  to  their  maximum  of  sensation. 

ward  or  inward  over  the  retinal  surface  appears  bordered  with  colors  which 
vary  with  the  rate  of  rotation  of  the  disk  and  with  the  amount  of  exhaustion 
of  the  retina.  Chromatic  effects  due  to  a  similar  cause  are  also  to  be  seen 
when  a  disk,  such  as  is  shown  in  Figure  242,  B  (known  as  Benham's  spectrum 


790  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

top),  is  rotated  with  moderate  rapidity.  The  concentric  bands  of  color  appear 
in  reverse  order  when  the  direction  of  rotation  is  reversed.  The  apparent 
movement  of  colored  figures  on  a  background  of  a  different  color  when  the 
eye  moves  rapidly  over  the  object  or  the  object  is  moved  rapidly  before  the 
eye  seems  to  depend  upon  this  same  retinal  peculiarity.  The  phenomenon 
may  be  best  observed  when  small  pieces  of  bright-red  paper  are  fastened  upon 
a  bright-blue  sheet  and  the  sheet  gently  shaken  before  the  eyes.  The  red 
figures  will  appear  to  move  upon  the  blue  background.  The  effect  may  be 
best  observed  in  a  dimly-lighted  room. 

In  this  connection  should  be  mentioned  the  phenomenon  of  "  recurrent 
images  "  or  "  oscillatory  activity  of  the  retina."  l  This  may  be  best  observed 
when  a  black  disk  containing  a  white  sector  is  rotated  at  a  rate  of  about  one 
revolution  in  two  seconds.  If  the  disk  is  brightly  illuminated,  as  by  sunlight, 

and  the  eye  fixed  steadily  upon  the  axis  of  rota- 
tion, the  moving  white  sector  seems  to  have  a 
shadow  upon  it  a  short  distance  behind  its  ad- 
vancing border,  and  this  shadow  may  be  followed 
by  a  second  fainter,  and  even  by  a  third  still 
fainter  shadow,  as  shown  in  Figure  243.  The 
distance  of  the  shadows  from  each  other  and 
from  the  edge  of  the  sector  increases  with  the  rate 
of  rotation  of  the  disk  and  corresponds  to  a  time 
FIG.  243,-To  illustrate  the  oscillatory  interval  of  about  0.01 5".  It  thus  appears  that 

activity  of  the  retina  (Charpentier).          , 

wnen  light  is  suddenly  thrown  upon  the  retina 

the  sensation  does  not  at  once  rise  to  its  maximum,  but  reaches  this  point  by 
a  sort  of  vibratory  movement.  The  apparent  duplication  of  a  single  very 
brief  retinal  stimulation,  as  that  caused  by  a  flash  of  lightning,  may  perhaps 
be  a  phenomenon  of  the  same  sort. 

Fatigue  of  Retina. — When  the  eye  rests  steadily  upon  a  uniformly  illu- 
minated white  surface  (e.  g.  a  sheet  of  white  paper),  we  are  usually  unconscious 
of  any  diminution  in  the  intensity  of  the  sensation,  but  it  can  be  shown  that 
the  longer  we  look  at  the  paper  the  less  brilliant  it  appears,  or,  in  other  words, 
that  the  retina  really  becomes  fatigued.  To  do  this  it  is  only  necessary  to  place 
a  disk  of  black  paper  on  the  white  surface  and  to  keep  the  eyes  steadily  fixed 
for  about  half  a  minute  upon  the  centre  of  the  disk.  Upon  removing  the  disk 
without  changing  the  direction  of  the  eyes  a  round  spot  will  be  seen  on  the 
white  paper  in  the  place  previously  occupied  by  the  disk.  On  this  spot  the 
whiteness  of  the  paper  will  appear  much  more  intense  than  on  the  neighboring 
portion  of  the  sheet,  because  we  are  able  in  this  experiment  to  bring  into  direct 
contrast  the  sensations  produced  by  a  given  amount  of  light  upon  a  fresh  and 
a  fatigued  portion  of  the  retina.2 

1  Charpentier:  Archives  de  Physiologic,  1892,  pp.  541,  629;  and  1896,  p.  677. 

2  Although  the  retina  is  here  spoken  of  as  the  portion  of  the  visual  apparatus  subject  to 
fatigue,  it  should  be  borne  in  mind  that  we  cannot,  in  the  present  state  of  our  knowledge,  dis- 
criminate between  retinal  fatigue  and  exhaustion  of  the  visual  nerve-centres. 


THE  SENSE    OF    VISION.  791 

The  rapidity  with  which  the  retina  becomes  fatigued  varies  with  the  color 
of  the  light.  Hence  when  intense  white  light  falls  upon  the  retina,  as  when 
we  look  at  the  setting  sun,  its  disk  seems  to  undergo  changes  of  color  as  one 
or  another  of  the  constituents  of  its  light  becomes,  through  fatigue,  less  and 
less  conspicuous  in  the  combination  of  rays  which  produces  the  sensation  of 
white. 

The  After-effect  of  Stimulation. — The  persistence  of  the  sensation  after  the 
stimulus  has  ceased  causes  very  brief  illuminations  (e.  g.  by  an  electric  spark)  to 
produce  distinct  effects.  On  this  phenomenon  depends  also  the  above-described 
method  of  mixing  colors  on  a  revolving  disk,  since  a  second  color  is  thrown 
upon  the  retina  before  the  impression  produced  by  the  first  color  has  had  time 
enough  to  become  sensibly  diminished.  The  interval  at  which  successive  stim- 
ulations must  follow  each  other  in  order  to  pro- 
duce a  uniform  sensation  (a  process  analogous 
to  the  tetanic  stimulation  of  a  muscle)  may  be 
determined  by  rotating  a  disk,  such  as  repre- 
sented in  Figure  244,  and  ascertaining  at  what 
speed  the  various  rings  produce  a  uniform  sen- 
sation of  gray.  The  interval  varies  with  the 
intensity  of  the  illumination  from  0.1 "  to 
0.033".  The  duration  of  the  after-effect  de- 
pends also  upon  the  length  of  the  stimulation 
and  upon  the  color  of  the  light  producing  it, 
the  most  persistent  effect  being  produced  by  the  FIG.  244,-Disk  to  illustrate  the  persistence 

of  retinal  sensation  (Helmholtz). 

red  rays.    In  this  connection  it  is  interesting  to 

note  that  while  with  the  rapidly  vibrating  blue  rays  a  less  intense  illumination 
suffices  to  stimulate  the  eye,  the  slowly  vibrating  red  rays  produce  the  more 
permanent  impression. 

After-images. — When  the  object  looked  at  is  very  brightly  illuminated  the 
impression  upon  the  retina  may  be  so  persistent  that  the  form  and  color  of  the 
object  are  distinctly  visible  for  a  considerable  time  after  the  stimulus  has  ceased 
to  act.  This  appearance  is  known  as  a  "  positive  after-image,"  and  can  be  best 
observed  when  we  close  the  eyes  after  looking  at  the  sun  or  other  bright  source 
of  light.  Under  these  circumstances  we  perceive  a  brilliant  spot  of  light  which, 
owing  to  the  above-mentioned  difference  in  the  persistence  of  the  impressions 
produced  by  the  various  colored  rays,  rapidly  changes  its  color,  passing  gen- 
erally through  bluish  green,  blue,  violet,  purple,  and  red,  and  then  disappear- 
ing. This  phenomenon  is  apt  to  be  associated  with  or  followed  by  another 
effect  known  as  a  "  negative  after-image."  This  form  of  after-image  is  much 
more  readily  observed  than  the  positive  variety,  and  seems  to  depend  upon  the 
fatigue  of  the  retina.  It  is  distinguished  from  the  positive  after-image  by  the 
fact  that  its  color  is  always  complementary  to  that  of  the  object  causing  it.  In 
the  experiment  to  demonstrate  the  fatigue  of  the  retina,  described  on  p.  790, 
the  white  spot  which  appears  after  the  black  disk  is  withdrawn  is  the  "  nega- 
tive after-image "  of  the  disk,  white  being  complementary  to  black.  If  a 


792  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

colored  disk  be  placed  upon  a  sheet  of  white  paper,  looked  at  attentively  for  a 
few  seconds,  and  then  withdrawn,  the  eye  will  perceive  in  its  place  a  spot  of 
light  of  a  color  complementary  to  that  of  the  disk.  If,  for  example,  the  disk 
be  yellow,  the  yellow-perceiving  elements  of  the  retina  become  fatigued  in 
looking  at  it.  Therefore  when  the  mixed  rays  constituting  white  light  are 
thrown  upon  the  portion  of  the  retina  which  is  thus  fatigued,  those  rays  which 
produce  the  sensation  of  yellow  will  produce  less  effect  than  the  other  rays  for 
which  the  eye  has  not  been  fatigued.  Hence  white  light  to  an  eye  fatigued  for 
yellow  will  appear  blue. 

If  the  experiment  be  made  with  a  yellow  disk  resting  on  a  sheet  of  blue 
paper,  the  negative  after-image  will  be  a  spot  on  which  the  blue  color  will 
appear  (1)  more  intense  than  on  the  neighboring  portions  of  the  sheet,  owing 
to  the  blue-perceiving  elements  of  that  portion  of  the  retina  not  being  fatigued  ; 
(2)  more  saturated,  owing  to  the  yellow-perceiving  elements  being  so  far 
exhausted  that  they  no  longer  respond  to  the  slight  stimulation  which  is  pro- 
duced when  light  of  a  complementary  color  is  thrown  upon  them,  as  has  been 
explained  in  connection  with  the  subject  of  saturation. 

Contrast. — As  the  eye  wanders  from  one  part  of  the  field  of  vision  to 
another  it  is  evident  that  the  sensation  produced  by  a  given  portion  of  the 
field  will  be  modified  by  the  amount  of  fatigue  produced  by  that  portion  on 
which  the  eye  has  last  rested,  or,  other  words,  the  sensation  will  be  the  result 


FIG.  245.— To  illustrate  the  phenomenon  of  contrast. 


of  the  stimulation  by  the  object  looked  at  combined  with  the  negative  after- 
image of  the  object  previously  observed.  The  effect  of  this  combination  is  to 
produce  the  phenomenon  of  successive  contrast,  the  principle  of  which  may  be 
thus  stated  :  Every  part  of  the  field  of  vision  appears  lighter  near  a  darker 


THE  SENSE    OF    VISION.  793 

part  and  darker  near  a  lighter  part,  and  its  color  seen  near  another  color 
approaches  the  complementary  color  of  the  latter.  A  contrast  phenomenon 
similar  in  its  effects  to  that  above  described  may  be  produced  under  conditions 
in  which  negative  after-images  can  play  no  part.  This  kind  of  contrast  is 
known  as  simultaneous  contrast,  and  may  perhaps  be  explained  on  the  theory 
that  a  stimulation  of  a  given  portion  of  the  retina  produces  in  the  neighboring 
portions  an  effect  to  some  extent  antagonistic  to  that  caused  by  direct  stimulation. 

A  good  illustration  of  the  phenomenon  of  contrast  is  given  in  Figure  245, 
in  which  black  squares  are  separated  by  white  bands  which  at  their  points  of 
intersection  appear  darker  than  where  they  are  bordered  on  either  side  by  the 
black  squares. 

A  black  disk  on  a  yellow  background  seen  through  white  tissue-paper 
appears  blue,  since  the  white  paper  makes  the  black  disk  look  gray  and  the 
yellow  background  pale  yellow.  The  gray  disk  in  contrast  to  the  pale  yellow 
around  it  appears  blue. 

The  phenomenon  of  colored  shadows  also  illustrates  the  principle  of  con- 
trast. These  may  be  observed  whenever  an  object  of  suitable  size  and  shape 
is  placed  upon  a  sheet  of  white  paper  and  illuminated  from  one  direction  by 
daylight  and  from  another  by  gaslight.  Two  shadows  will  be  produced,  one 
of  which  will  appear  yellow,  since  it  is  illuminated  only  by  the  yellowish  gas- 
light, while  the  other,  though  illuminated  by  the  white  light  of  day,  will 
appear  blue  in  contrast  to  the  yellowish  light  around  it. 

Space-perception. — Rays  of  light  proceeding  from  every  point  in  the 
field  of  vision  are  refracted  to  and  stimulate  a  definite  point  on  the  sur- 
face of  the  retina,  thus  furnishing  us  with  a  local  sign  by  which  we  can 
recognize  the  position  of  the  point  from  which  the  light  proceeds. 
Hence  the  size  and  shape  of  an  optical  image  upon  the  retina  enable  us  to 
judge  of  the  size  of  the  corresponding  object  in  the  same  way  that  the  cutane- 
ous terminations  of  the  nerves  of  touch  enable  us  to  judge  of  the  size  and 
shape  of  an  object  brought  in  contact  with  the  skin.  This  spatial  perception 
is  materially  aided  by  the  muscular  sense  of  the  muscles  moving  the  eyeball, 
for  we  can  obtain  a  much  more  accurate  idea  of  the  size  of  an  object  if 
we  let  the  eye  rest  in  succession  upon  its  different  parts  than  if  we  gaze  fixedly 
at  a  given  point  upon  its  surface.  The  conscious  effort  associated  with  a  given 
amount  of  muscular  motion  gives,  in  the  case  of  the  eye,  a  measure  of  distance 
similar  to  that  secured  by  the  hand  when  we  move  the  fingers  over  the  surface 
of  an  object  to  obtain  an  idea  of  its  size  and  shape. 

The  perception  of  space  by  the  retina  is  limited  to  space  in  two  dimensions 
— i.  e.  in  a  plane  perpendicular  to  the  axis  of  vision.  Of  the  third  dimension 
in  space — i.  e.  of  distance  from  the  eye — the  retinal  image  gives  us  no  know- 
ledge, as  may  be  proved  by  the  study  of  after-images.  If  an  after-image  of 
any  bright  object — e.  g.  a  window — be  produced  upon  the  retina  in  the  man- 
ner above  described  and  the  eye  be  then  directed  to  a  sheet  of  paper  held  in 
the  hand,  the  object  will  appear  outlined  in  miniature  upon  the  surface  of  the 
paper.  If,  however,  the  eye  be  directed  to  the  ceiling  of  the  room,  the  object 


794  AN  AMERICAN   1EXT-BOOK    OF  PHYSIOLOGY. 

will  appear  enlarged  and  at  a  distance  corresponding  to  that  of  the  surface 
looked  at.  Hence  one  and  the  same  retinal  image  may,  under  different  cir- 
cumstances, give  rise  to  the  impression  of  objects  at  different  distances.  We 
must  therefore  regard  the  perception  of  distance  not  as  a  direct  datum  of  vision, 
but,  as  will  be  later  explained,  a  matter  of  visual  judgment. 

When  objects  are  of  such  a  shape  that  their  images  may  be  thrown  suc- 
cessively upon  the  same  part  of  the  retina,  it  is  possible  to  judge  of  their  rela- 
tive size  with  considerable  accuracy,  the  retinal  surface  serving  as  a  scale  to 
which  the  images  are  successively  applied.  When  this  is  not  the  case,  the 
error  of  judgment  is  much  greater.  We  can  compare,  for  instance,  the  relative 
length  of  two  vertical  or  of  two  horizontal  lines  with  a  good  deal  of  precision,, 
but  in  comparing  a  vertical  with  a  horizontal  line  we  are  liable  to  make  a  con- 
siderable error.  Thus  it  is  difficult  to  realize  that  the  vertical  and  the  hori- 
zontal lines  in  Figure  246  are  of  the  same  length.  The  error  consists  in  an 

over-estimation  of  the  length  of  the  vertical 
lines  relatively  to  horizontal  ones,  and  appears  to 
depend,  in  part  at  any  rate,  upon  the  small  size 
of  the  superior  rectus  muscle  relatively  to  the 
other  muscles  of  tKeTeye.  The  difference  amounts 
to  30-45  per  cent,  in  weight  and  40-53  per  cent, 
in  area  of  cross  section.  It  is  evident,  therefore, 
that  a  given  motion  of  the  eye  in  the  upward 
direction  will  require  a  more  powerful  contraction 
of  the  weaker  muscle  concerned  in  the  movement 


FIG.  246.-To  illustrate  the  over-esti-    than  will  be  demanded  of  the  stronger  muscles 

mation  of  vertical  lines. 

moving  the  eye  laterally  to   an    equal    amount. 

Hence  we  judge  the  upward  motion  of  the  eye  to  be  greater  because  to  accom- 
plish it  we  make  a  greater  effort  than  is  required 
for  a  horizontal  movement  of  equal  extent. 

The  position  of  the  vertical  line  bisecting  the 
horizontal  one  (in  Fig.  246)  aids  the  illusion,  as 
may  be  seen  by  turning  the  page  through  90°,  so 
as  to  bring  the  bisected  line  into  a  vertical  posi- 
tion, or  by  looking  at  the  lines  in  Figure  247,  in 
which  the  illusion  is  much  less  marked  than  in 
Figure  246. 

The  tendency  to  over-estimate  the  length  of 
vertical  lines  is  also  illustrated  by  the  error 
commonly  made  in  supposing  the  height  of  the 
crown  of  an  ordinary  silk  hat  to  be  greater 

,1          .,      i          1,1  FIG.  247.— To  illustrate  the  over-estima- 

than  its  breadth.  tion  of  vertical  lines. 

Irradiation.  —  Many    other    circumstances 

affect  the  accuracy  of  the  spatial  perception  of  the  retina.  One  of  the  most 
important  of  these  is  the  intensity  of  the  illumination.  All  brilliantly  illumi- 
nated objects  appear  larger  than  feebly  illuminated  ones  of  the  same  size,  as  is 


THE  SENSE    OF    VISION.  795 

well  shown  by  the  ordinary  incandescent  electric  lamp,  the  delicate  filament  of 
which  is  scarcely  visible  when  cold,  but  when  intensely  heated  by  the  electric 
current  glows  as  a  broad  band  of  light.  The  phenomenon  is  known  as  "  irra- 
diation/7 and  seems  to  depend  chiefly  upon  the  above-described  imperfections 
in  the  dioptric  apparatus  of  the  eye,  in  consequence  of  which  points  of  light 
produce  small  circles  of  dispersion  on  the  retina  and  bright  objects  produce 


FIG.  248.— To  illustrate  the  phenomenon  of  irradiation. 

images  with  imperfectly  defined  outlines.  The  white  square  surrounded  by 
black  and  the  black  square  surrounded  by  white  (Figure  248),  being  of  the 
same  size,  would  in  an  ideally  perfect  eye  produce  images  of  the  same  size  on 
the  retina,  but  owing  to  the  imperfections  of  the  eye  the  images  are  not  sharply 


FIG.  249.— To  illustrate  the  phenomenon  of  irradiation. 

defined,  and  the  white  surfaces  consequently  appear  to  encroach  upon  the  darker 
portions  of  the  field  of  vision.  Hence  the  white  square  looks  larger  than  the 
black  one,  the  difference  in  the  apparent  size  depending  upon  the  intensity  of 
the  illumination  and  upon  the  accuracy  with  which  the  eye  can  be  accommo- 


796 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


dated  for  the  distance  at  which  the  objects  are  viewed.  The  effect  of  irradi- 
ation is  most  manifest  when  the  dark  portion  of  the  field  of  vision  over  which 
the  irradiation  takes  place  has  a  considerable  breadth.  Thus  the  circular  white 
spots  in  Figure  249,  when  viewed  from  a  distance  of  three  or  four  meters, 
appear  hexagonal,  since  the  irradiation  is  most  marked  into  the  triangular  dark 
space  between  three  adjacent  circles.  A  familiar  example  of  the  effect  of  irra- 
diation is  afforded  by  the  appearance  of  the  new  moon,  whose  sun-illuminated 
•crescent  seems  to  be  part  of  a  much  larger  circle  than  the  remainder  of  the 
disk,  which  shines  only  by  the  light  reflected  upon  it  from  the  surface  of  the 


Subdivided  Space.  —  A  space  subdivided  into  smaller  portions  by  inter- 
mediate objects  seems  more  extensive  than  a  space  of  the  same  size  not  so  sub- 
divided. Thus  the  distance  from  A  to  B  (Fig.  250)  seems  longer  than  that  from 


D  E 

FIG.  250.— To  illustrate  the  illusion  of  subdivided  space. 

B  to  C,  though  both  are  of  the  same  length,  and  for  the  same  reason  the  square 
D  seems  higher  than  it  is  broad,  and  the  square  E  broader  than  it  is  high,  the 
illusion  being  more  marked  in  the  case  of  D  than  in  the  case  of  E,  because,  as 
above  explained,  vertical  distances  are,  as  a  rule,  over-estimated. 

The  explanation  of  this  illusion  seems  to  be  that  the  eye  in  passing  over  a 
subdivided  line  or  area  recognizes  the  number  and  size  of  the  subdivisions, 
and  thus  gets  an  impression  of  greater  total  size  than  when  no  subdivisions 
are  present. 

A  good  example  of  this  phenomenon  is  afforded  by  the  apparently  increased 
extent  of  a  meadow  when  the  grass  growing  on  it  is  cut  and  arranged  in  hay- 
cocks.1 

The  relations  of  lines  to  each  other  gives  rise  to  numerous  illusions  of 
spatial  perception,  among  the  most  striking  of  which  are  those  afforded  by  the 
so-called  "  Zollner's  lines,"  an  example  of  which  is  given  in  Figure  251.  Here 

1  It  is  interesting  to  note  that  a  similar  illusion  has  been  observed  when  an  interval  of  time 
subdivided  by  audible  signals  is  compared  with  an  equal  interval  not  so  subdivided  (Hall  and 
Jastrow  :  Mind,  xi.  62). 


THE  SENSE    OF    VISION.  797 

the  horizontal  lines,  though  strictly  parallel  to  each  other,  seem  to  diverge  and 
converge  alternately,  their  apparent  direction  being  changed  toward  greater  per- 

xxx  xx  xx  xx  xx 

X  X  X  XX  XX  X  X  X  X 

\\\\\  \\x\\ 


\\  \  \  \  \  \  \  \  \  \ 
XXXXXXXXXXX 

XX  X  X  X  XX  XX  XX 

xwwwwx  v 

\\N\\N\\\\\ 

FIG.  251.—  Zollner's  lines. 

pendicularity  to  the  short  oblique  lines  crossing  them.     This  illusion  is  to  be  ex- 

plained in  part  by  the  tendency  of  the  eye  to  over-estimate  the  size  of  acute  and  fo- 

under-estimate that  of  obtuse  angles  —  a  tendency  which 

also  affords  a  partial  explanation  of  the  illusion  in 

Figure  252,  where  the  line  d  is  the  real  and  the  line/ 

the  apparent  continuation  of  the  line  a.     The  illusion 

in  Zollner's  figures  is  more  marked  when  the  figure  is 

so  held  that  the  long  parallel  lines  make  an  angle  of 

about  45°  with  the  horizon,  since  in  this  position  the 

eye  appreciates  their  real  position  less  accurately  than 

when  they  are  vertical  or  horizontal.     It  is  dimin- 

ished, but  does  not  disappear,  when  the  eye,  instead 

of  being  allowed  to  wander  over  the  figure,  is  fixed 

upon  any  one  point  of  the  field  of  vision.     Hence  the 

J   .    .    '  ,    ,  „  .       .          FIG.  252,-To  illustrate  illusion 

motions  of  the  eye  must  be  regarded  as  a  factor  in,  but          Of  space-perception. 
not  the  sole  cause  of,  the  illusion. 

Our  estimate  of  the  size  of  given  lines,  angles,  and  areas  is  influenced  by 
neighboring  lines,  angles,  and  areas  with  which  they  are  compared.  This 
influence  is  sometimes  exerted  in  accordance  with  the  principle  of  contrast, 
and  tends  to  make  a  given  extension  appear  larger  in  presence  of  a  smaller, 


FIG.  253.— To  illustrate  contrast  in  space-perception  (Muller-Lyer). 

and  smaller  in  presence  of  a  larger  extension.  This  effect  is  illustrated  in 
Figure  253,  in  which  the  middle  portion  of  the  shorter  line  appears  larger 
than  the  corresponding  portion  of  the  longer  line,  in  Figure  254,  in  which  a 
similar  effect  is  observed  in  the  case  of  angles,  and  in  Figure  255,  in  which 


798  AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 

the  space  between  the  two  squares  seems  smaller  than  that  between  the  two 
oblong  figures. 

In  some  case,  however,  an  influence  of  the  opposite  sort l  seems  to   be 


FIG.  254.— To  illustrate  contrast  in  space-perception  (Muller-Lyer). 

exerted,  as  is  shown  in  Figure  256,  in  which  the  middle  one  of  three  parallel 
lines  seems  longer  when  the  outside  lines  are  longer,  and  shorter  when  they 
are  shorter  than  it  is  itself,  and  in  Figure  257,  where  a  circle  appears  larger 


FIG.  255.— To  illustrate  contrast  in  space-perception  (Muller-Lyer). 

if  surrounded  by  a  circle  larger  than  itself,  and  smaller  if  a  smaller  circle  is 
shown  concentrically  within  it. 

Lines  meeting  at  an  angle  appear  longer  when  the  included  angle  is  large 


FIG.  256.— To  illustrate  so-called  "  confluxion  "  in  space-perception  (Muller-Lyer). 

than  when  it  is  small,  as  is  shown  in  Figure  258.  This  influence  of  the 
included  angle  affords  a  partial  explanation  of  the  illusion  shown  in  Figure 
259,  where  the  horizontal  line  at  B  seems  longer  than  at  A  ;  but  the  distance 

1  For  this  influence  the  name  "confluxion"  has  been  proposed  by  Muller-Lyer,  from  whose 
article  in  the  Archiv  fur  Physiologic,  1889,  Sup.  Bd.,  the  above  examples  are  taken. 


THE   SENSE    OF    VISION. 


799 


between  the  extremities  of  the  oblique  lines  seems  also  to  affect  our  estimate 
of  the  horizontal  line  in  the  same  way  as  the  outside  lines  in  Figure  256 
influence  our  judgment  of  the  length  of  the  line  between  them. 

Perception  of  Distance. — The  retinal  image  gives  us,  as  we  have  seen, 
no  direct  information   as  to  the  distance  of  the  object  from  the  eye.     This 


FIG.  257.— To  illustrate  so-called  "  confluxion  "  in  space-perception  (Miiller-Lyer). 

knowledge  is,  however,  quite  as  important  as  that  of  position  in  a  plane  per- 
pendicular to  the  line  of  vision,  and  we  must  now  consider  in  what  way  it  is 
obtained.  The  first  fact  to  be  noted  is  that  there  is  a  close  connection  between 
the  judgments  of  distance  and  of  actual  size.  A  retinal  image  of  a  given 
size  may  be  produced  by  a  small  object  near  the  eye  or  by  a  large  one  at  a 


FIG.  258.— To  illustrate  the  influence  of  angles  upon  the  apparent  length  of  lines  (Mviller-Lyer). 

distance  from  it.  Hence  when  we  know  the  actual  size  of  any  object  (as,  for 
example,  a,  human  figure)  we  judge  of  its  distance  by  the  size  of  its  image  on 
the  retina.  Conversely,  our  estimate  of  the  actual  size  of  an  object  will 
depend  upon  our  judgment  of  its  distance.  The  fact  that  children  constantly 
misjudge  both  the  size  and  distance  of  objects  shows  that  the  knowledge  of 


FIG.  259.— Illusion  of  space-perception. 

this  relation  is  acquired  only  by  experience.  If  circumstances  mislead  us 
with  regard  to  the  distance  of  an  object,  we  necessarily  make  a  corresponding 
error  with  regard  to  its  size.  Thus,  objects  seen  indistinctly,  as  through  a  fog, 
are  judged  to  be  larger,  because  we  suppose  them  to  be  farther  off,  than  they 
really  are.  The  familiar  fact  that  the  moon  seems  to  be  larger  when  near  the 
horizon  than  when  near  the  zenith  is  also  an  illustration  of  this  form  of  illu- 


800  AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 

sion.  When  the  raoon  is  high  above  our  heads  we  have  no  means  of  esti- 
mating its  distance  from  us,  since  there  are  no  intervening  objects  with  which 
we  can  compare  it.  Hence  we  judge  it  to  be  nearer  than  when,  seen  on  the 
horizon,  it  is  obviously  farther  off  than  all  terrestrial  objects.  Since  the  size 
of  the  retinal  image  of  the  moon  is  the  same  in  the  two  cases,  we  reconcile 
the  sensation  with  its  apparent  greater  distance  when  seen  on  the  horizon  by 
attributing  to  the  moon  in  this  position  a  greater  actual  size. 

If  the  retinal  image  have  the  form  of  a  familiar  object  of  regular  shape — 
e.  g.  a  house  or  a  table — we  interpret  its  outlines  in  the  light  of  experience 
and  distinguish  without  difficulty  between  the  nearer  and  more  remote  parts  of 
the  object.  Even  the  projection  of  the  outlines  of  such  an  object  on  to  a  plane 
surface  (i.  e.  a  perspective  drawing)  suggests  the  real  relations  of  the  different 
parts  of  the  picture  so  strongly  that  we  recognize  at  once  the  relative  distances 
of  the  various  portions  of  the  object  represented.  How  powerfully  a  familiar 
outline  can  suggest  the  form  and  relief  usually  associated  with  it  is  well  illus- 
trated by  the  experiment  of  looking  into  a  mask  painted  on  its  interior  to 
resemble  a  human  face.  In  this  case  the  familiar  outlines  of  a  human  face 
are  brought  into  unfamiliar  association  with  a  receding  instead  of  a  projecting 
form,  but  the  ordinary  association  of  these  outlines  is  strong  enough  to  force 
the  eye  to  see  the  hollow  mask  as  a  projecting  face.1  rThe  fact  that  the  pro- 
jecting portions  of  an  object  are  usually  more  brightly  illuminated  than  the 
receding  or  depressed  portions  is  of  great  assistance  in  determining  their  rela- 
tive distance.  This  use  of  shadows  as  an  aid  to  the  perception  of  relief  pre- 
supposes a  knowledge  of  the  direction  from  which  the  light  falls  on  an  object, 
and  if  we  are  deceived  on  the  latter  we  draw  erroneous  conclusions  with 
regard  to  the  former  point.  Thus,  if  we  look  at  an  embossed  letter  or  figure 
through  a  lens  which  makes  it  appear  inverted  the  accompanying  reversal  of 
the  shadows  will  cause  the  letter  to  appear  depressed.  The  influence  of 
shadows  on  our  judgment  of  relief  is,  however,  not  so  strong  as  that  of  the 
outline  of  a  familiar  object.  In  a  case  of  conflicting  testimony  the  latter 
usually  prevails,  as,  for  example,  in  the  above-mentioned  experiment  with  the 
mask. 

Aided  by  these  peculiarities  of  the  retinal  picture,  the  mind  interprets  it  as 
corresponding  in  its  different  parts  to  points  at  different  distances  from  the  eye, 
and  it  is  interesting  to  notice  that  painters,  whose  work,  being  on  a  plane  sur- 
face, is  necessarily  in  all  its  parts  at  the  same  distance  from  the  eye,  use  similar 
devices  in  order  to  give  depth  to  their  pictures.  Distant  hills  are  painted  with 
indistinct  outlines  to  secure  what  is  called  "  aerial  perspective."  Figures  of 
men  and  animals  are  introduced  in  appropriate  dimensions  to  suggest  the  dis- 
tance between  the  foreground  and  the  background  of  the  picture.  Landscapes 
are  painted  preferably  by  morning  and  evening  light,  since  at  these  hours  the 
marked  shadows  aid  materially  in  the  suggestion  of  distance. 

1  In  the  experiment  the  mask  should  be  placed  at  a  distance  of  about  two  meters  and  one 
eye  closed.  Even  with  both  eyes  open  the  illusion  often  persists  if  the  distance  is  increased  to 
five  or  six  meters. 


THE  SENSE    OF    VISION.  801 

The  eye,  however,  can  aid  itself  in  the  perception  of  depth  in  ways  which 
the  painter  has  not  at  his  disposal.  By  the  sense  of  effort  associated  with  the 
act  of  accommodation  we  are  able  to  estimate  roughly  the  relative  distance  of 
objects  before  us.  This  aid  to  our  judgment  can,  of  course,  be  employed  only 
in  the  case  of  objects  comparatively  near  the  eye.  Its  effectiveness  is  greater 
for  objects  not  far  from  the  near-point  of  vision,  and  diminishes  rapidly  as  the 
distance  is  increased,  and  disappears  for  distances  more  than  two  or  three  meters 
from  the  eye. 

When  the  head  is  moved  from  side  to  side  an  apparent  change  in  the  rela- 
tive position  of  objects  at  different  distances  is  produced,  and,  as  the  extent  of 
this  change  is  inversely  proportional  to  the  distance  of  the  objects,  it  serves  as 
a  measure  of  distance.  This  method  of  obtaining  the  "  parallax  "  of  objects 
by  a  motion  of  the  head  is  often  noticeable  in  persons  whose  vision  in  one 
eye  is  absent  or  defective. 

Binocular  Vision. — The  same  result  which  is  secured  by  the  comparison 
of  retinal  images  seen  successively  from  slightly  different  points  of  view  is 
obtained  by  the  comparison  of  the  images  formed  simultaneously  by  any  object 
in  the  two  eyes.  In  binocular  vision  we  obtain  a  much  more  accurate  idea  of 
the  shape  and  distance  of  objects  around  us  than  is  possible  with  monocular 
vision,  as  may  be  proved  by  trying  to  touch  objects  in  our  neighborhood  with 
a  crooked  stick,  first  with  both  eyes  open  and  then  with  one  eye  shut.  When- 
ever we  look  at  a  near  solid  object  with  two  eyes,  the  right  eye  sees  farther 
round  the  object  on  the  right  side  and  the  left  eye  farther  round  on  the  left. 
The  mental  comparison  of  these  two  slightly  different  images  produces  the 
perception  of  solidity  or  depth,  since  experience  has  taught  us  that  those  objects 
only  which  have  depth  or  solidity  can  affect  the  eyes  in  this  way.  Conversely, 
if  two  drawings  or  photographs  differing  from  each  other  in  the  same  way  that 
the  two  retinal  images  of  a  solid  object  differ  from  each  other  are  presented, 
one  to  the  right  and  the  other  to  the  left  eye,  the  two  images  will  become 
blended  in  the  mind  and  the  perception  of  solidity  will  result.  Upon  this  fact 
depends  the  effect  of  the  instrument  known  as  the(  stereoscope^  the  slides  of 
which  are  generally  pairs  of  photographs  of  natural  objects  taken  simultaneous- 


FIG.  260.— To  illustrate  stereoscopic  vision. 


ly  with  a  double  camera,  of  which  the  lenses  are  at  a  distance  from  each  other 
equal  to  or  slightly  exceeding  that  between  the  two  axes  of  vision.  The  prin- 
ciple of  the  stereoscope  can  be  illustrated  in  a  very  simple  manner  by  drawing 
circles  such  as  are  represented  in  Figure  260  on  thin  paper,  and  fastening  each 

51 


802 


AN  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


pair  across  the  end  of  a  piece  of  brass  tube  about  one  inch  or  more  in  diameter 
and  ton  inches  long.  Let  the  tubes  be  held  one  in  front  of  each  eye  with  the 
distant  ends  nearly  in  contact  with  each  other,  as  shown  in  Figure  261.  If 
the  tubes  are  in  such  a  position  that  the  small  circles  are  brought  as  near  to 
Jeach  other  as  possible,  as  shown  in  Figure  260,  th§  retinal  images  will  blend, 

the  smaller  circle  will  seem  to  be  much 
nearer  than  the  larger  one,  and  the  eyes  will 
appear  to  be  looking  down  upon  a  truncated 
cone,  such  as  is  shown  in  Figure  262,  since 
a  solid  body  of  this  form  is  the  only  one 


Fio.  261.— To  illustrate  stereoscopic  vision. 


FIG.  262.— To  illustrate  stereoscopic  vision. 


bounded  by  circles  related  to  each  other  as  those  shown  in  this  experiment. 

Stereoscopic  slides  often  serve  well  to  illustrate  the  superiority  of  binocular 
over  monocular  vision.  If  the  slide  represents  an  irregular  mass  of  rocks  or 
ice,  it  is  often  very  difficult  by  looking  at  either  of  the  pictures  by  itself  to 
determine  the  relative  distance  of  the  various  objects  represented,  but  if  the 
slide  is  placed  in  the  stereoscope  the  true  relation  of  the  different  parts  of  the 
picture  becomes  at  once  apparent. 

Since  the  comparison  of  two  slightly  dissimilar  images  received  on  the  two 
retinas  is  the  essential  condition  of  stereoscopic  vision,  it  is  evident  that  if  the 
two  pictures  are  identical  no  sensation  of  relief  can  be  produced.  Thus,  when 
two  pages  printed  from  the  same  type  or  two  engravings  printed  from  the  same 
plate  are  united  in  a  stereoscope,  the  combined  picture  appears  as  flat  as  either 
of  its  components.  If,  however,  one  of  the  pictures  is  copied  from  the  other, 
«ven  if  the  copy  be  carefully  executed,  there  will  be  slight  differences  in  the 
distances  between  the  lines  or  in  the  spacing  of  the  letters  which  will  cause 
apparent  irregularities  of  level  in  the  different  portions  of  the  combined  pic- 
ture. Thus,  a  suspected  banknote  may  be  proved  to  be  a  counterfeit  if,  when 
placed  in  a  stereoscope  by  the  side  of  a  genuine  note,  the  resulting  combined 
picture  shows  certain  letters  lying  apparently  on  different  planes  from  the  rest. 

Pseudoscopic  Vision.—  If  the  pictures  of  an  ordinary  stereoscopic  slide  be 


reversed^  so  that  the  picture  bplong''"g  1>n 


the  left  eye,  and 


presented  to 

Drives  place  to  what  is  called  a  pseudo- 


right 


scopic  effect — /.  c.  we  perceive  not  a  solid  but  a  hallow  body.    The  effect  is  best 


THE  SENSE    OF    VISION.  803 

vl 

obtained  with  the  outlines  of  geometrical  solids,  photographs  of  corns  or  medals 
or  of  objects  'which  may  readily  exist  ir^  an  inyerted  fo^m.  ..IKfafifff  the  photo- 
graphs represent  objects  which  cannot  be  thus  inverted,  such  as  buildings  and 
landscapes,  the  pseudoscopic  effect  is  not  readily  produced—  another  example 
of  the  power  (see  p.  800)  of  the  outline  of  a  familiar  object  to  outweigh  other 
snrfo  of 


A  pseudoscopic  effect  may  be  readily  obtained  without  the  use  of  a  stereo- 
scope by  simply  converging  the  visual  axes  so  that  the  right  eye  looks  at  the 
left  and  the  left  eye  at  the  right  picture  of-  a  stereoscopic  slide.  The  eyes  may 
hfi_aided  in.  assuming  the  right  degree  of  convergence,  by  looking  at  a  small 
object  like  the  head  of  a  pin  held  between  the  eyes  and  the  slide  in  the  manner 
described  on  p.  758.  Figure  260,  viewed  in  this  way,  will  present  the  appear- 
ance of  a  hollow  truncated  cone  with  the  base  turned  toward  the  observer.  A 
stereoscopic  slide  with  its  pictures  reversed  will,  of  courser  when  viewed  in  this 
way,  present  not  a  pseudoscopic,  but  a  true  stereoscopic,  appearance,  as  shown 
by  Figures  226  and  227. 

Binocular  Combination  of  Colors.  —  The  effect  of  binocnlarly  combin- 
ing two  different  colors  varies  with  the  difference  in  wave-length  of  the  colors. 
Colors  lying  near  each  other  in  the  spectrum  will  generally  blend  together 
and  produce  the  sensation  of  a  mixed  color,  such  as  would  result  from  the 
union  of  colors  by  means  of  the  revolving  disk  or  by  the  method  of  reflected 
and  transmitted  light,  as  above  described.  Thus  a  red  and  a  yellow  disk 
placed  in  a  stereoscope  may  be  generally  combined  to  produce  the  sensation 
of  orange.  If,  however,  the  colors  are  complementary  to  each  other,  as  blue 
and  yellow,  no  such  mixing  occurs,  but  the  field  of  vision  seems  to  be  occupied 
alternately  by  a  blue  and  by  a  yellow  color.  This  so-called  "  rivalry  of  the 
fields  of  vision  "  seems  to  depend,  to  a  certain  extent,  upon  the  fact  that  in 
order  to  see  the  different  colors  with  equal  distinctness  the  eyes  must  be  dif- 
ferently accommodated,  for  it  is  found  that  if  the  colors  are  placed  at  different 
distances  from  the  eyes  (the  colors  with  the  less  refrangible  rays  being  at  the 
greater  distance),  the  rivalry  tends  to  disappear  and  the  mixed  color  is  more 
easily  produced. 

An  interesting  effect  of  the  stereoscopic  combination  of  a  black  and  a 
white  object  is  the  production  of  the  appearance  of  a  metallic  lustre  or  polish. 
If,  for  instance,  the  two  pictures  of  a  stereoscopic  slide  represent  the  slightly 
different  outlines  of  a  geometrical  solid,  one  in  black  upon  white  ground  and 
the  other  in  white  upon  black  ground,  their  combination  in  the  stereoscope 
will  produce  the  effect  of  a  solid  body  having  a  smooth  lustrous  surface. 
The  explanation  of  this  effect  is  to  be  found  in  the  fact  that  a  polished  surface 
reflects  the  light  differently  to  the  two  eyes,  a  given  point  appearing  bril- 
liantly illuminated  to  one  eye  and  dark  to  the  other.  Hence  the  stereoscopic 
combination  of  black  and  white  is  interpreted  as  indicating  a  polished  surface, 
since  it  is  by  means  of  a  polished  surface  that  this  effect  is  usually  produced. 

Corresponding1  Points.  —  When  the  visual  axes  of  both  eyes  are  directed 
to  the  same  object  two  distinct  images  of  that  object  are  formed  upon  widely 


804  ^Vr  AMERICAN   TEXT-BOOK   OF  PHYSIOLOGY. 


separated  parts  of  the  nervous  system.  Yet  but  a  single  object  is  perceived. 
The  phenomenon  is  the  same  as  that  which  occurs  when  a  grain  of  sand  is 
held  between  the  thumb  and  finger.  In  both  cases  we  have  learned  (chiefly 
through  the  agency  of  muscular  movements  and  the  nerves  of  muscular  sense) 
to  interpret  the  double  sensation  as  produced  by  a  single  object. 

Any  two  points,  lying  one  in  each  retina,  the  stimulation  of  which  by  rays 
of  light  gives  rise  to  the  sensation  of  light  proceeding  from  a  single  object  are 
said  to  be  "  corresponding  points."  Now,  it  is  evident  that  thefovece  centrales 
of  the  two  eyes  must  be  corresponding  points,  for  an  object  always  appears 
single  when  both  eyes  are  fixed  upon  it.  That  double  vision  results  when  the 
images  are  formed  on  points  which  are  not  corresponding  may  be  best  illus- 
trated by  looking  at  three  pins  stuck  in  a  straight  rod  at  distances  of  35,  45, 
and  55  centimeters  from  the  end.  If  the  end  of  the  rod  is  held  against  the 
nose  and  the  eyes  directed  to  each  of  the  three  pins  in  succession,  it  will  be 
found  that,  while  the  pin  looked  at  appears  single,  each  of  the  others  appears 
double,  and  that  the  three  pins  therefore  look  like  five. 

The  two  fovece  centrales  are  not,  of  course,  the  only  corresponding  points. 
In  fact,  it  may  be  said  that  the  two  retinas  correspond  to  each  other,  point  for 
point,  almost  as  if  they  were  superposed  one  upon  the  other  with  the  fovese 
together.  The  exact  position  of  the  points  in  space  which  are  projected  on  to 
corresponding  points  of  the  two  retinas  varies  with  the  position  of  the  eyes. 
The  line  or  surface  in  which  such  points  lie  is  known  as  the  "  horopter."  A 
full  discussion  of  the  horopter  would  be  out  of  place  in  this  connection,  but 
one  interesting  result  of  its  study  may  be  pointed  out  —  viz.  the  demonstration 
that  when,  standing  upright,  we  direct  our  eyes  to  the  horizon  the  horopter  is 
approximately  a  plane  coinciding  with  the  ground  on  which  we  stand.  It  is 
of  course  important  for  security  in  walking  that  all  objects  on  the  ground 
should  appear  single,  and,  as  they  are  known  by  experience  to  be  single,  the 
eye  has  apparently  learned  to  see  them  so. 

Since  the  vertical  meridians  of  the  two  eyes  represent  approximately  rows 
of  corresponding  points,  it  is  evident  that  when  two  lines  are  so  situated  that 
their  images  are  formed  each  upon  a  vertical  meridian  of  one  of  the  eyes,  the 
impression  of  a  single  vertical  line  will  be  produced,  for  such  a  line  seen  bin- 
ocularly  is  the  most  frequent  cause  of  this  sort  of  retinal  stimulation.  This 
is  the  explanation  commonly  given  of  the  singular  optical  illusion  which  is 
produced  when  lines  drawn  as  in  Figure  263  are  looked  at  with  both  eyes  fixed 
upon  the  point  of  intersection  of  the  lines  and  with  the  plane  in  which  the 
visual  axes  lie  forming  an  angle  of  about  20°  with  that  of  the  paper,  the  dis- 
tance of  the  lines  from  the  eyes  being  such  that  each  line  will  lie  approximately 
in  the  same  vertical  plane  with  one  of  the  visual  axes.  Under  these  circum- 
stances each  line  will  form  its  image  on  a  vertical  meridian  of  one  of  the  eyes, 
and  the  combination  of  these  images  results  in  the  perception  of  a  third  line, 
not  lying  in  the  plane  of  the  paper,  but  apparently  passing  through  it  more  or 
less  vertically,  and  swinging  round  its  middle  point  with  every  movement  of 
the  head  or  the  paper.  In  this  experiment  it  will  be  found  that  the  illusion 


THE  SENSE    OF    VISION. 


805 


of  a  line  placed  vertically  to  the  plane  of  the  paper  does  not  entirely  dis- 
appear when  one  eye  is  closed.     Hence  it  is  evident  that  there  is,  as  Mrs. 


FIG.  264.— Monocular  illusion  of  vertical  lines. 

C.  L.  Franklin  has  pointed  out,1  a  strong  tendency  to  regard 
lines  which  form  their  images  approximately  on  the  vertical 
meridian  of  the  eye  as  themselves  vertical.  This  tendency 
is  well  shown  when  a  number  of  short  lines  converging 
toward  a  point  outside  of  the  paper  on  which  they  are 
drawn,  as  in  Figure  264,  are  looked  at  with  one  eye  held 
a  short  distance  above  the  point  of  convergence.  Even 
when  the  lines  are  not  convergent,  but  parallel,  so  that  their 
images  cannot  fall  upon  the  vertical  meridian  of  the  eye,  the 
illusion  is  not  entirely  lost.  It  will  be  found,  for  instance, 
that  when  the  Zollner  lines,  as  given  in  Figure  251,  are 
looked  at  obliquely  with  one  eye  from  one  corner  of  the 
FIG.  263.— Binocu-  figure,  the  short  lines  which  lie  nearly  in  a  plane  with  the 
ticanine  visual  axis  appear  to  stand  vertically  to  the  plane  of  the 

paper. 

In  this  connection  it  may  be  well  to  allude  to  the  optical  illusion  in  conse- 
quence of  which  certain  portraits  seem  to  follow  the  beholder  with  the  eyes. 
This  depends  upon  the  fact  that  the  face  is  painted  looking  straight  out  from 
the  canvas  — i.  e.  with  the  pupil  in  the  middle  of  the  eye.  The  painting  being 
upon  a  flat  surface,  it  is  evident  that,  from  whatever  direction  the  picture  is 
viewed,  the  pupil  will  always  seem  to  be  in  the  middle  of  the  eye,  and  the 
eye  will  consequently  appear  to  be  directed  upon  the  observer.  The  phenom- 
enon is  still  more  striking  in  the  case  of  pictures  of  which  the  one  repre- 
sented in  Figure  265  may  be  taken  as  an  example.  Here  the  soldier's  rifle 

1  Am.  Journal  of  Psychology,  vol.  i.  p.  99. 


806 


AN  AMERICAN   TEXT-BOOK    OF  PHYSIOLOGY. 


FIG.  265.— Illusion  of  lines  always  pointing 
toward  observer. 


is  drawn  as  it  appears  to  an  eye  looking  straight  down  the  barrel,  and,  as  this 
foreshortening  is  the  same  in  all  positions  of  the  observer,  it  is  evident  that 

when  such  a  picture  is  hung  upon  the  wall 
of  a  room  the  soldier  will  appear  to  be 
aiming  directly  at  the  head  of  every  person 
present. 

In  concluding  this  brief  survey  of  some 
of  the  most  important  subjects  connected 
with  the  physiology  of  vision  it  is  well  to 
utter  a  word  of  caution  with  regard  to  a 
danger  connected  with  the  study  of  the  sub- 
ject. This  danger  arises  in  part  from  the 
fact  that  in  the  scientific  study  of  vision  it 
is  often  necessary  to  use  the  eyes  in  a  way 
quite  different  from  that  in  which  they  are 
habitually  employed,  and  more  likely,  there- 
fore, to  cause  nervous  and  muscular  fatigue. 
We  have  seen  that  in  any  given  position  of 
the  eye  distinct  definition  is  limited  to  an 
area  which  bears  a  very  small  proportion  to 

the  whole  field  of  vision.  Hence  in  order  to  obtain  an  accurate  idea  of  the 
appearance  of  any  large  object  our  eyes  must  wander  rapidly  over  its  whole 
surface,  and  we  use  our  eyes  so  instinctively  and  unconsciously  in  this  way 
that,  unless  our  attention  is  specially  directed  to  the  subject,  we  find  it  diffi- 
cult to  believe  that  the  power  of  distinct  vision  is  limited  to  such  a  small 
portion  of  the  retina.  In  most  of  the  experiments  in  physiological  optics, 
however,  this  rapid  change  of  direction  of  the  axis  of  vision  must  be  carefully 
avoided,  and  the  eye-muscles  held  immovable  in  tonic  contraction. 

Our  eyes,  moreover,  like  most  of  our  organs,  serve  us  best  when  we  do  not 
pay  too  much  attention  to  the  mechanism  by  which  their  results  are  brought 
about.  In  the  ordinary  use  of  the  eyes  we  are  accustomed  to  neglect  after- 
images, intraocular  images,  and  all  the  other  imperfections  of  our  visual  appa- 
ratus, and  the  usefulness  of  our  eyes  depends  very  much  upon  our  ability  thus 
to  neglect  their  defects.  Now,  the  habit  of  observing  and  examining  these 
defects  that  is  involved  in  the  scientific  study  of  the  eye  is  found  to  interfere 
with  our  ability  to  disregard  them.  A  student  of  the  physiology  of  vision 
who  devotes  too  much  attention  to  the  study  of  after-images,  for  instance,  may 
render  his  eyes  so  sensitive  to  these  phenomena  that  they  become  a  decided 
obstacle  to  ordinary  vision. 


